This is a very good question. It seems from Tosfos (Kesuvos 47a DH d'Masar) that they are obligated in Tosefes Shabbos and Yom Tov, and that the obligation is from the Torah. The Minchas Chinuch (Mitzvah #313:10), however, argues persuasively that only in regards to Tosefes Inuy on Yom Kippur are they obligated by the Torah. The Gemara (Sukah 28b) explicitly includes them in this Mitzvah by way of a Ribuy ("ha'Ezrach"). But no such Ribuy for women exists for Tosefes Shabbos and Yom Tov which is learned from the word Tishbesu (Yoma 81a). Shemiras Shabbos k'Hilchasa (II, 46:1) follows the Psak of Tosfos.

Kol Tuv,

Yonasan Sigler