https://www.gamespot.com/articles/hackers-already-added-more-games-to-switch-onlines/1100-6461898/

Apparently, hacking into Nintendo Switch Online and adding your own ROMs isn't as hard as you might think. In fact, it's easy enough that most people would be able to do it.

According to Polygon, Switch Online is so exploitable because the service is using the same type of easily hackable emulation software that Nintendo implemented in the NES Classic and SNES Classic consoles. One of the hackers who's already added their own ROMs to Nintendo Switch Online, kapu, told Polygon that setting everything up takes a bit of work, but is manageable with instructions. After that setup, it's "insanely easy to swap out ROMs and add entirely new ones."