I have an Existing outdoor power panel with a 200 amp breaker (not easily removable) and no more slots. I also own a 100 amp outdoor panel. I need temporary service to build my home My question is.. Can I hook up my 100 amp box to the 200 amp service. Or do I need a new 200 amp box? Thanks

Convert a few of the circuits of the 200 A panel to twin breakers. They put two circuits in a single space to make room for a 100 A breaker for the 100 A panel.