I'm in a similar league (6 keepers) but Yahoo recognized a 1st rounder as being in what would be the 7th round. Our commish took down all of our keepers for this season and I think he had a way of designating them as such, making sure they came off the board to each respective team before anyone had a chance to draft.
What I'm saying is I'm fairly sure you can modify the keepers at the start of next season and the picks will work out - all of my keepers from last year have a black "K" next to them now. Hope this helps!