That starts to make things complicated. Trying to tie merc companies and vassals and courtiers all together.Of course, barbarians inside need to give the emperor bonuses, like a mercenary company. A lot of barbarian leaders were referred to as kings, so I suppose you could make them petty kings with a mercenary company inside the empire if you didn't want them to be actual kings, and if you can't have landless counts then maybe they could just become courtiers.
What if the barbarians are vassalized as my above suggestion (granted if it works) and part of the WP gives the emperor event troops, like 2-5k of them?