Yes. You can merge them (Correct me if I'm wrong everyone else, on whatever I say).
You can do what you said, however you will also need to make sure you have the teams logo pictures in the file and edit teams.txt accordingly. Look at how it's laid out, and you'll need to add all the added teams data onto it. Poke around in the files and you should understand, it's not the most complicated of files. The chart I use to help/learn AI which I'm attaching below, made by Philetbabe, gives some great info on the teams.txt and AI stuff in general.