He probably can.

[Cold War Spook]

It will probably end up in the courts in all the border states and federal courts. I used probably twice because I am not an attorney. If the wall is built and in some places it has to be built any distance from the border, the land South of the wall will still be in the U.S. That means that people can walk up to the wall and say hello. They will be on U.S. soil thus they will be afforded the protection of The Constitution. Even now, there are about 1.1 million cases backlogged because there are less than 800 immigration judges. In some places, it could actually benefit people wanting to enter. Maybe they could put a phone on that side of the wall to call their lawyers.