Mexico is already divided into 31 states (and is, like the U.S., a federal republic of long-standing, so we’re not just talking about administrative regions or departments here); even if hell did freeze over, it would be a tough sell convincing the Mexicans they should for some reason give up their current states instead of just having them all admitted on equal terms to the new Union. (Mexico’s least populous state has a population about equal to that of our least populous state, Wyoming.) Even if you insisted on dividing Mexico into new states with populations all equal to the average state population in the U.S., we’re talking about another 18 states.
Put it another way, Mexico’s population is over a third that of the U.S. Definitely not another Alaska or Hawaii.