In a series of tweets, KapuccinoHeck explained that the Switch Online app that handles the official NES games treats them like plain .nes files, with a database file that lists in plaintext all compatible NES games, and which can be manipulated to add others. In other words, if the 20 classic NES games that come with Nintendo Switch Online don't float your boat, it's not impossible to add your favorites, as demonstrated in a video by KapuccinoHeck, where they loaded up Kirby's Adventure.
That said, this is all only applicable to hacked Switches. Hacking a Switch is a complicated, time-consuming job that could see your account banned by Nintendo, so it's not really advisable. However, KapuccinoHeck did mention that the NES app on the Switch appears to be a recreated port of the NES emulator running on the NES Classic. As such, it might not be too long until we see tools available to help people increase their NES library on the Switch's own emulator in a safer and easier way.