Nintendo hosted a bumper Direct on Tuesday including so much information that you might have missed the new N64 games announced for NSO's Expansion Pack. Pokemon Stadiums 1 and 2 were among them, but there was some bad news in the small print.
A small message appeared at the bottom of the screen revealing “Pokemon cannot be transferred to these games”. The original games allowed players to transfer their Pokemon teams from Game Boy titles to Stadium. You won't be able to do that on Switch.
Nintendo Life pointed out the small print after the fact, and Serebii's Joe Merrick has attempted to explain Nintendo's decision. In order for Game Boy teams to become usable in Stadium, the game needed to make frequent connections with the Transfer Pak on which the Pokemon were stored. That would mean the Switch would need to be constantly connected to a 3DS, a console some suggested could be a solution since the older Pokemon games are on it.
Merrick then reminded Pokemon fans online support for the 3DS will come to an end in March anyway. The thread goes on to explain why other suggested solutions probably wouldn't work, such as bringing Pokemon Home into the mix. The conclusion is that transfers between the Game Boy games and Stadium on NSO could be achieved in the future, but only if those original games are eventually added to the console's online library.
That's a very real possibility, of course, as there have been rumors Game Boy titles would be coming to NSO for over a year. They were actually expected when Nintendo announced the Expansion Pack, but the Genesis got a library instead. For now, you'll have to play Stadium without your trusty team of powerful Pokemon, but there is hope if Game Boy games are ever added to the Switch.