Assimilation is based off where the province is located. Any province in the Americas will fully assimilate any pop to your own culture so long as 2 conditions are met. The 1st is you need your own pop in that province for other pops to assimilate to. The 2nd is the pops that are assimilating must NOT have a core on that province. For example, if either UK or USA takes territory in Mexico, all Mexican pops in Mexico WON'T assimilate to Yankee or British. However, all Mayan pops in Mexico will assimilate to your American/British culture because Mayan doesn't have a core anywhere in Mexico. Do note that since Mayan doesn't have a core in Mexico, this means that the Mayan culture that exists in southern Mexico at the start of the game will quickly turn into 100% Mexican in a few years.
Also note that assimilation is a completely separate mechanic from immigration. In Vicky 2 the two are entirely unrelated to each other. It's the location of the province, not the controller, that determines assimilation rate. The only exception to this is some events and triggered modifiers that buff assimilation temporarily. American/Oceania provinces have something like a 200% bonus (or more, haven't played vanilla in awhile) to assimilation.
AFAIK there's no mod that changes this (its not historical or balanced to have US assimilating Germans in Germany) but if you wanted to mod it yourself you could just add a triggered modifier to USA that gives a bonus to assimilation big enough to counter the penalty from POPs having a core in a province.
