It has already been reported that the NES Classic Edition has officially been hacked in order to allow the installation of new games, with the mod allowing for more than double the number of titles to be played. The hack originally appeared on a Russian forum, and then later on Reddit.
There are currently two ways to make the modification happen; you can either go with the original, complicated process of modifying the firmware, or the somewhat easier-but-riskier option of downloading a do-it-all tool that will do all the hard work for you. Here's a rundown of both.
The Long Way
A Reddit user by the name of Xelonakias took the original instructions away from the creator, MadMonkey, and translated them from native Russian to English. The result is a very wordy, very complicated, but extremely detailed and thorough guide. It is good to remember that doing this could potentially result in the malfunctioning of your NES Classic and completely ruining an already amazing gadget. Continue at your own risk. Full step-by-step instructions are found here.
The Easier-but-riskier Way
If you're second-guessing on your modding abilities after reading the massive blocks of text from 'The Long Way,' you should be aware that there's a much more straightforward way of adding games to the NES Classic, and it will not be requiring a Mario save slot or any other complicated nonsense. This hack was created by a Russian modder named Cluster, who made the tool available to everyone via a recent forum post.
The tool appears to be legitimate in that it actually works 100%, and many Reddit users have reported that they were able to successfully add games to their Classic Edition using nothing more than a USB cable and, of course, the NES game ROMs - which you can find anywhere, via Google. But despite it being easy, you also need to keep in mind that it can also potentially endanger both the PC that you are using and the NES console itself.
So you still need to be careful despite it being a straightforward approach.