A convert's biological parents are not their parents in a halachic sense. In some senses the converting beis din is "responsible" for the convert in the way of parents. In name, at least, the ger's parents are Avrohom Avinu and Sarah Imenu. When, if ever, does a convert say yizkor?
As a convert, I've only asked the question with respect to kaddish and shiva. I've posted the answers I've received on those questions elsewhere (in sum, my rav relying on the Rema said "no kaddish or shiva" but there are other chashuva sources who disagree; I go with my Rav). Since my late Rav, Rabbi Gedaliah Anemer, zt'l, said "no" to those questions, it never occurred to me to ask about Yizkor. But I do know that with respect to my Jewish-born wife, Rabbi Anemer (and his student and successor, Rabbi Dovid Rosenbaum, shlita), said she could not say Yizkor until her parent had been dead one year. Given that I never bothered asking about Yizkor. I don't know any converts who stay around for Yizkor, and I don't either.