Bottom line: Thanks to Nintendo being "lazy," gamers with hacked Switches can add their own ROMs to the Switch's NES app. Nintendo is probably watching and waiting with the ban hammer, however, so proceed with caution.
Nintendo formally launched its Nintendo Switch Online service on September 18. In addition to facilitating online play and offering up cloud saves, the service affords access to a curated selection of classic games.
The online library launched with 20 NES games and Nintendo promises that more will be added on a regular basis. For those that simply can’t wait that long (and are willing to take some risks), it’s possible to upload your own ROMs and play them through the NES app on the Switch.
good lord #NintendoSwitchOnline's NES games are plain .nes files— kapu | the Gay. (@KapuccinoHeck) September 19, 2018
GUI appears to be entirely LUA much like the S/NES Classic so people are gonna be able to go to town on this
The "database" file that contains a listing of all valid games is plaintext,- pic.twitter.com/bO8M4B7U5e
According to recent posts from Twitter user @KapuccinoHeck, the Switch’s NES app uses plain .nes files and the emulator works very similar to the one used on the NES Classic Edition. The app also uses a database file that lists compatible NES games in plaintext.
In other words (and as demonstrated), it’s not all that difficult to add your own ROMs to the mix.
That said, KapuccinoHeck advises against it. The technique can only be pulled off on a hacked Switch which is described by Kotaku as a complex undertaking that carries a constant threat of having your account banned by Nintendo. Furthermore, KapuccinoHeck has been told that a lot of data from the app is being sent to Nintendo and they have a few legitimacy checks that occur at random.
If Nintendo spots any fishy business, odds are, they’ll take action and you won’t like the results.