I mean, I've seen it on multiple sites and I'm hoping it will happen?
After all, I think I've seen the sinnoh starters on the national pokedex and you meet Crasher Wake...
Is it possible?
No, you cannot. The only time in a Pokemon game where you could travel to another region was in Gold, Silver, Crystal, HeartGold, and SoulSilver, when you can travel to Kanto from Johto.