Copy all the Imperium Romanum files into the main Brytenwalda folder.
File A = Your main fold, the one you find in the modules with all the other mods you may have for Warband.
File B = the backup file, usually the one people might save incase they ever want to play the origonal?
What you're going to do is very simple. Copy/paste then rename the folder Imperium Romanum. Once you're done, go to wherever you have the OTHER file, the origonal Brytenwalda and copy it back into the modules folder for Warband. Thus, you have both Imperium Romanum and Brytenwalda.