Rashi on Deuteronomy 20:18/רש"י על דברים כ׳:י״ח

1למען אשר לא ילמדו [BUT THOU SHALT DOOM THEM TO DESTRUCTION …] THAT THEY TEACH YOU NOT TO DO [AFTER THEIR ABOMINATIONS] — Consequently if they repent of their abominations and wish to become proselytes you are allowed to accept them as such (Sifrei Devarim 202:1).