You can use different sets of enabled addons for boxing vs solo, sure. You can set up a Virtual File called Addons.txt, assign it to each of your Characters. You don't need several different filenames for this, you can virtualize it as the same name for all of your guys, like "Addons-isboxer.txt"

Then you can just use the Addons menu like normal at char select to turn different ones on/off, and the virtual file will keep it separated when you're using ISBoxer. enjoy