Copying games will be fairly tough yes. The games have certain banks assigned, but they can also share data between each other on rare occasions.
With that aside though, the way you describe it makes it sound really possible (and I think it really is). Problem is that the actual coding work requires more than you may think. Dealing with long jumps will be the main problem because you'll have to locate every single one and make them point to the 'new game' you just added, as well as adding the 'new game' to the menu also, which requires some ASM hacking. After that, you'll have to identify what banks are used exactly and just 'copy' them over in such a way that they are used by that new game alone, so you have maximum control over it without breaking anything else.
. I could post stuff now and then