From a within-universe perspective, you are correct. They could modify the appearance of Dahj/Soji to look like anything. Though the question would of course be why bother. Clearly in this case they did not make the decision themselves as to their appearance after all.
From a show perspective, if Dahj/Soji are meant to be the biological children of Jurati and Maddox, the show would want to hint this in some manner. As of yet, it has not, which is why I do not suspect this to be the case. It does look we'll get to see who Soji believed her dad to be via dream sequence next week, but these were implanted memories anyway, so there's no reason to think this person is any more real than the "Mom AI" - regardless, it doesn't look much like Maddox.