What is the significance of bringing converts to Har Tziyon?
Radak: Like one brings a guest to his house and receives him b'Simchah, so He will command the Kohanim 1 to accept them b'Simchah when they come to convert.
Malbim: There they will learn Torah and Emunah - "Ki mi'Tziyon Tetzei Sorah u'Dvar Hashem mi'Yrushalayim" (2:3).
Also a Beis Din of Yisraelim can accept converts. Perhaps Radak specified Kohanim, for they were normally Chachamim. Or, convert would approach the Kohanim, for they do the Avodah, and are most likely to deal with conversion. (PF)
How will He make them rejoice in His house of Tefilah?
Radak: They will rejoice when they see themselves in the Azarah, which is a Beis Tefilah every year, with Yisrael, in their prayers, Olos and Zevachim.
What is the significance of Olos and Zevachim?
Malbim: They will not bring Korbanos for sin, rather, voluntarily.
Will His house be for all nations?
Rashi: It is even for converts; it is not only for Yisrael. Malbim - this shows that converts are not separated from Yisrael!
Radak: Yes. This is like Shlomo said "v'Gam El ha'Nochri" (Melachim I, 8:41) - all the more so for those who return to the creed of Yisrael!
