As I mentioned before, Christians have been grafted in.[Rom 11:17, 23-24 KJV] 17 And if some of the branches be broken off, and thou, being a wild olive tree, wert graffed in among them, and with them partakest of the root and fatness of the olive tree; … 23 And they also, if they abide not still in unbelief, shall be graffed in: for God is able to graff them in again. 24 For if thou wert cut out of the olive tree which is wild by nature, and wert graffed contrary to nature into a good olive tree: how much more shall these, which be the natural [branches], be graffed into their own olive tree?
There is no requirement that Gentiles are excluded from being grafted in. As a matter of fact, there is a precedence of non-Jews in the OT that folded into the Jewish community.
Rahab[Jos 6:25 KJV] 25 And Joshua saved Rahab the harlot alive, and her father’s household, and all that she had; and she dwelleth in Israel [even] unto this day; because she hid the messengers, which Joshua sent to spy out Jericho.
Ruth[Rth 1:4 KJV] 4 And they took them wives of the women of Moab; the name of the one [was] Orpah, and the name of the other Ruth: and they dwelled there about ten years.