“YehudahTzvi – One who raises a child is as if he bore him. The meforshim say this on countless occasions, even in places where the Torah uses terms like ??. See, for example, Ibn Ezra to Bereishis 46:7. “
So if I adopt a Yisroel kid he can duchan? I don’t think so. Yichus, and thus Ben David is genetic.