“If a Moabitess converts to Judaism can she be the grandmother of a King of Israel and have the Messiah as her descendant ?”
Sure. But conversion to Judaism didn’t grant her a personal claim to the Holy Land. She was a stranger who married into the royal family of the Biblical chosen people. She herself was a foreigner with no special heritage or ancestral membership to the chosen people. And that by no means demonstrates that a person who converted to Judaism after the coming of Christ shares in God’s Old Covenant promise to Abraham.