The Nintendo Switch Online NES app went live earlier this week and, within about a day, the application was hacked to allow installation of pretty much any other NES game you want. I'd strongly caution against doing this yourself, though, as the consequences for these actions might be severe.
The folks over at Polygon picked up on some tweets from user Kapu, who has broken down the process by which just about anyone with a little time on their hands can hack their Switch's NES collection to download their own ROMs onto the application. Before going any further, it should probably be stated that pirating is a crime and any ROMS you're using should only be digital copies of games you actually own. Also, I'd put money on tampering with Nintendo's NES app being a big no-no in the EULA you agreed to simply by using a Nintendo Switch, and it will also void your warranty, so, again, it's probably best if you just take this at its face value as interesting news and maybe don't go and try to replicate the process.
Anyway, Kapu hopped onto the Switch's NES app and discovered that the setup is "incredibly basic." In fact, he notes that the ROMS used in the application are just standard .nes files rather than the altered files used in Virtual Console games like on the Wii U and 3DS. For those latter consoles, you had to do some extra finagling to get your ROMS to actually run. For the Switch, though, your most basic of ROMS should work just fine.
As Kapu notes, the setup and files are pretty much identical to the ones seen with the NES Classic and SNES Classic, which folks also cracked open with a quickness and modded to play additional ROMS. Kapu states that this is likely not an act of laziness on Nintendo's part or even a potential vulnerability so much as a specific design choice to make it easier for the company to add new (legitimate) games to the NES library.
When Nintendo Switch Online launched this week, it included a collection of 20 NES games such as Super Mario Bros. 3 and Balloon Fight. Another nine games will launch for the library over the next three months, making it look like Nintendo's plan, at least for the time being, is to put out three games a month for the service. This is all included in your Switch Online subscription fee, which goes for four bucks a month or $20 for a full year.
The initial report includes a warning from Kapu that folks should maybe not try to do this themselves, as there's no telling what Nintendo's security capabilities/reactions will be at this point. We know for a fact that the NES collection needs to be connected to the internet at least once a week to make sure you're still a subscribing member of the Switch Online service. There's a very good chance Nintendo will be able to tell if you've been tampering with your NES collection during these mandatory check-ins, and there's nothing stopping them from disabling your account or worse.
So, once again, it's an interesting note that the Switch's NES library is so easy to break into, but doing so could be a very risky business.