Hackers have successfully broken into the NES Classic Edition and have managed to allow users to add their own games to the library. First appearing on the Russian gaming community GBX, if a user was to successfully complete the step-by-step tutorial they'd even be able to add homebrewed games to their NES Classic Edition.
It's important to note that the instructions are far from simple, however, and besides being written in Russian they detail how you're required to get into the recovery (FEL) mode), do some funny stuff with the kernel, and then some even sketchier stuff with custom ROMs. Luckily some English video tutorials have been made which at least help neutralize the language barrier. It's unclear if the process may damage your device and there is no guarantee that any specific game you would try to load onto your NES Classic Edition will eventually work.
Over on Reddit the people behind the /r/nesclassicmods subreddit have done a malware scan on the various tools required for the hack and have found some issues which they believe are false positives. Some redditors like /u/zenmechanic have gone forward with the hack and managed to install a number of different games including TMNT 2, Tetris, Tiny Toons, and... if you were wondering... Battletoads.