Or HaChaim on Exodus 12:48/אור החיים על שמות י״ב:מ״ח

1 וכי יגור אתך גר, "When a stranger will sojourn with you, etc." This was Moses' authority to accept converts from amongst the Egyptians. The reason the Torah writes אתך, "with you" (sing), is that such conversions had to be personally approved by Moses. There is also a hint here that G'd agreed to this only reluctantly, i.e. לרצון משה, and that is why the Torah wrote אתך גר, "he is a stranger (proselyte) with you, (but not with Me). G'd's reluctance was due to Moses' inability to examine G'd's hidden reasons. We all know what the result of Moses' accepting such proselytes was and how they triggered the sin of the golden calf.

2המול לו כל זכר, "let all his males be circumcised, etc." Just as failure to circumcise a slave prevents such a slave from participating in the Passover, so failure of a proselyte to circumcise all his male slaves is a prerequisite before the proselyte himself can eat of the Passover. The reason that the Torah adds ואז יקרב, "and then he may approach," is to tell us that he does not need to appoint a natural born Jew to slaughter his Passover offering but that he may do so himself. This is why the Torah adds: "he will be just like a natural born Jew."