Actually, here's my final solution.
(1) Ask all the Central Americans to vote to dissolve their governments, adopt the U.S. Constitution, and peacefully join the United States. However, they cannot send representatives to Congress until after the next Census.
(2) Right now, nearly 10% of Mexico's population lives here. Therefore, simply annex Mexico since, apparently, it's every Mexican's dream to be an American because their own gov'ts are too inept or corrupt. (After all, we don't have this problem w/Canada. Right?)
Seems like common sense to me.