[Kekkonen]

I'm not sure the in-game help had anything on resupply. I agree it would be a little odd if you couldn't do it at your protectorate -- "We expect you guys to fight and die for us, but please go somewhere else for gas" doesn't sound like something you'd say to an empire several times more powerful than yours, be it friendly or not. (Of course, I could always colonize a small planet or a moon in their system and make it a supply depot, but I assume this will piss them off.)