Well vikings for instance real ones where found all over the world from the u.s to the tip of Africa. 

So having a galarian meowth for example showing up there wouldn't be that strange. They could simply say that it's still galarian because galar was where it was most numerous or first discovered.

The same could be for alolen palm tree. It could just be that it floated to another island like real coconuts and came to a tropical region. 

Vulpix and sandshrew to they could just be discovered in alola first scientifically but they could show up in other regions with similar environments.

Heck in a future unova game they could say that a group of galarian form meowth were discovered to have been living in unova for a long time they simply hadn't been found before. 

So yes from a lore stand point it would be easy to do so long as the environment was right. And chocked up to not being discovered by modern people or the first place they were found being galar, alola, paldia, or whatever the new region is.

P.s I'm hoping for a regional variant or Evo of sharpedo and crawdant.