It's possible, I've done it in my mod. However I don't really remember exactly how I did it.
I think the limit is set by event so you'll have to edit the .sct-files. You might also want to add some names for the new armies to 1-USA.fac and 2-CSA.fac
After thinking about it, I think the number of armies are regulated by how many commanders are actually able to form armies. This is done via model files for leaders. Add the line "CanBeGHQStack = 1" to enable army creation for a specific leader.