A couple who could not have children naturally had kids by means of a surrogate (not sure whose sperm or egg or if that’s relevant). They’re having a giyur (conversion) for the kid and have requested to use their names, and not Avraham/Sara on the teudat giyur (conversion certificate). Do you have thoughts/an opinion about this? Thank you in advance!
If it were both the husband’s sperm and the wife’s egg then you definitely can, because many authorities say that maternity is defined by whose egg it was (for example, Rabbi Shlomo Goren (Torat HaRefuah, pg. 176-177), Rabbi Ovadia Yosef (HaRishon LeTzion 1 Even HaEzer 5), Rabbi Shlomo Amar (Shema Shlomo Even HaEzer 8:1-3), Rabbi Yaakov Ariel (Technumin 16 5756, 171, etc.).
Although I personally incline towards the position that it goes by who is carrying the fetus (see for example Tzitz Eliezer 19:40, Rabbi Zalman Nechemia Goldberg (Techumin 5, 258), Rav Mordechai Eliyahu, etc.), I think that we can rely on this position for this matter.
If it was not the husband’s sperm, but still the wife’s egg then, in line with the above, you could still use the mother’s name.
If it was the father’s sperm and not the mother’s egg and it was the egg of a non-Jew then the husband would not be halakhically related to the fetus.
All of that is a matter of the halakhic status. However, as far as writing names on the conversion certificate or a ketubah, my practice in such cases is to write the father’s name and then המג׳ (= המגדלו) and the same with the mother’s name.
I would call him up for an Aliyah with the father’s name (and the Mother’s name if that’s your practice) without worrying about any addition/qualifier because what really matters there is just people know who the name is referring to.