He could just establish that Israeli law applies to the existing settlements as if they were annexed. He might make it more secure by stating that the national government (not the settlement) would be fully liable if a future peace treaty requires paying the Palestinians for the lands, or relocating the settlement (insure the settlements against an unfavorable future peace treaty).
Israel has to remember that if the Republicans ever lose a national election in the United States, they will be, as the saying up, rowing up creek without a paddle.