Yeah, you'd have to partially repeal posse comitatus to pull that off. I'll assume that's part of the question and vote "yes." Limited repeal, of course.
Posse comitatus wouldn't have to be repealed. All Congress would need to do is authorize the use of troops in this instance. The posse law is not as restrictive as many believe, and it was changed substantially in 1981 so the military could assist the police and Coast Guard in responding to drug smuggling. There are drugs coming across the southern border as we speak, I'm sure.
No it would not. The Posse Commitatus act is more limited than you might think. If a governor requests federal help due to a breakdown in public order, the military can be sent in, as they were during the Rodney King riots.
But these are federal laws that would be enforced, but the military would not be enforcing them anyway, in the sense of arresting people for trial. They'd be pushing them back across the border, or just blocking them from crossing in the first place and repelling any armed incursions by the Mexican gangs, military or police, including the Federales (Federal Judical Police). If arrests were necessary, the Border Patrol could put liaison officers with the military units, and they could make the arrests.
Or do you think the federal military can't resist an intrusion on US territory, armed or not, without an act of Congress? Posse Commitatus does not go that far. By your notion, the Destroyer which fired the first shots of WW-II (for the US, in the Pacific) was violating the Act. War had not been declared, so I guess they should have sent for federal marshalls when they spotted the mini-sub trying to sneak into Pearl Harbor. (BTW, the hit the sub and presumably caused it to sink. Not long ago the sub was found with a nice hole, of the same size as the destroyer's gun, in it's conning tower. :) )