There should be a manned gate every 100 to 200 miles where illegals immigrants can go to become legal citizens again.
They call those outposts embassies and consulates. They can do just like everybody else in the world does and apply at a consulate or embassy.
If they had been good neighbors like the Canadians (I know this will get some Canada Haters on FR going, but they are good neighbors) we wouldn't need a fence. Fences MAKE good neighbors.