There are some patchers out there, like Neoncube, Triad Patcher, Jikari Patcher, Thor patcher, KPatcher, etcetera. If you want to use an already made patcher, you could go get some insight to the Patchers category in our Wiki.
If you wish to make your own parcher from scratch, you'll need to know more about the grf file structure and so own, which I myself don't know, so good luck if that's your case.