I have worked part time at several different places while keeping a full time thing going. (which doesn't pay much, but I choose my own adventure) Each one has tried to make me sign paperwork involving them getting first right of refusal on things I make. I have refused to sign those documents, three times now, twice for one company, with no real recourse from their side. HR put on a show but I didn't take the bait. Make a stand when it comes to things like that. I own my spare time, nobody else.