[Translated from Hebrew:]
Regarding what you wrote in Insights to Yevamos 46a, see Teshuvos Har Tzvi (Yoreh Deah 220), who writes as follows: "Regarding the question of a prospective convert who wants to convert but the doctors have said that it is not possible to give him Milah because of his weak heart... it is obvious that the case is not similar to that of a convert who is 'Nichras ha'Gid' and thus has no Orlah at all and is not included in the category of 'Arel.' In our case, though, where the person has an Orlah but is just unable to do Milah because of the danger involved, it stands to reason that he has a status of an 'Arel' for all matters. We find with regard to eating Terumah that an Arel who cannot do Milah to himself is the subject of a Machlokes between Rashi and Rabeinu Tam, see Yevamos 70a, and see Tosfos in Chagigah (4b) and Zevachim (22b). According to Rashi and many other Rishonim, an Arel who cannot do Milah because of the danger, such as an Arel whose brothers died from Milah, is considered an Arel for all matters. Rabeinu Tam argues with Rashi and maintains that an Arel whose brothers died from Milah, and thus he cannot do Milah because of the danger, is not considered an Arel and is permitted to eat Terumah. However, even according to Rabeinu Tam, it is not possible to accept him as a convert in such a case, because the view of Rabeinu Tam is that a Ger who is already circumsized when he converts has no Takanah (and cannot become a Ger)....
Nonetheless, it is possible to argue based on the Gemara in Yevamos regarding a Ger who did Tevilah but did not do Milah. Rebbi Elazar and Rebbi Yehoshua hold... that he is a Ger, for so we find that the women at the time of Matan Torah converted with Tevilah alone. Rebbi Yosi, whom the Halachah follows, holds that he is not a Ger, until he does both Milah and Tevilah, and we do not learn from the women at Matan Torah, because we do not judge that which is possible from that which is not possible. According to this, in a case where the Ger is unable to do Milah because of the danger, it seems that he would be called a case of 'not possible,' and his case would be able to be learned from the case of women at Matan Torah, that Tevilah alone suffices for his Gerus.
However, in my opinion, it appears that there is still room to argue and to say that just like we do not judge the the possible from the impossible, so, too, we do not judge that which is possible in actuality (doing Milah to a man, even though it is dangerous) from that which is not possible in actuality (doing Milah to a woman)...."
See also Sh'eilos u'Teshuvos Melamed l'Ho'il (Yoreh Deah 86), Seridei Eish (2:102-103; 3:96), Da'as Kohen (150), Be'er Moshe (8:126), and Tzitz Eliezer (14:92, and 15:1-12).
b'Birkas ha'Torah,
Yeshayah Dovid Kaye
Thank you very much for bringing to our attention the opinion of Rabeinu Tam, which was not mentioned in our Insights (in Yevamos 46b, and in Shabbos 135b).
(a) The opinion cited in the name of Rabeinu Tam is actually the opinion of Rabeinu Chananel, cited by the Tur (beginning of YD 268), from the Ramban, Rashba, and Ritva in Shabbos (135a). The Bach changes the text of the Tur to read "Rabeinu Tam" instead of "Rabeinu Chananel," as the Nimukei Yosef records the source, based on what Tosfos in Yevamos (46b) writes in the name of Rabeinu Chananel that seems to conflict with this opinion. However, we can see clearly that the printing error is in the Nimukei Yosef, and not in the Tur and in the majority of Rishonim that quote this opinion, for we find this opinion written explicitly in the Rabeinu Chananel in Shabbos 135b (except that a printing error omitted a few words there). When the Bach cites Tosfos in Yevamos, anyone who reads Tosfos there will see that Tosfos is quoting that Rabeinu Chananel.
Thus, we have corrected our Insights (included below) to reflect this view of Rabeinu Chananel.
(b) Regarding the Teshuvah of Ha'Gaon Rav Tzvi Pesach Frank, who writes that a Nochri who is already Mahul cannot convert, while one who is Nichras ha'Gid may convert, this is very questionable according to the opinion of Rabeinu Chananel. It seems clear that according to Rabeinu Chananel, neither of them can become converts, and similarly one whose brothers died from Milah cannot convert. Anyone who wants to convert but does not do Milah -- for any reason that might be -- he has no Takanah to become a Ger.
Mordecai Kornfeld
(c) TOSFOS (46b, DH d'Rebbi Yosi, Shabbos 135a DH Lo Nechleku ) quotes RABEINU CHANANEL who rules that, even today, any Nochri who comes to convert and already has a Milah does not need Hatafas Dam Bris in order to become a Ger. He seems to hold that even though their Milah was entirely secular, since it cannot physically be done again for Geirus, he does not need a Milah for Geirus. Even though other Rishonim hold that Hatafas Dam Bris comes in place of Milah even for a Ger, Rabeinu Chananel seems to argue. He maintains that it is done only for a child born with a Milah; since that was the way he was born, there is a different form of Milah for him -- Hatafas Dam Bris. For one who was born normal, the only valid form of Milah is the removal of the Orlah, and since he had his Orlah removed already, he cannot have another Milah. (See Insights to Shabbos 135:2.) This would also answer why the tribe of Levi did not need a new Milah for Geirus.
(Actually, although from the way he is quoted in Tosfos this seems to be Rabeinu Chananel's opinion, actually Rabeinu Chananel seems to be saying something else entirely (see Rabeinu Chananel in Shabbos 135b, where a few words seem to be missing in the version printed in the Vilna Shas, and see how Rabeinu Chananel is cited by the RAMBAN and Rishonim as well as by the TUR (YD 268)). Rabeinu Chananel writes that because there is no Hatafas Dam Bris for a convert, a non-Jew that is already circumcised cannot become a convert ! (Rabeinu Chananel enigmatically adds that although his conversion does not entitle him to become accepted as a Jew, nevertheless children born to him afterwards are Jews!) None of the Rishonim concur with him in this regard, though.)