Is the Borg Boff a character unlock? In that case you should be able to send him to another character or sell it on the exchange.
These officers are usually unique, so only one of teh same type can be used per character.
However you can use a Borg Engineer and a Borg Science officer at the same time.
The thing is, I bought 2 of them with Zen. Not from an unlock. They were just called Liberated Borg Bridge Officers. So I used one of them at first and made them a Borg Engineer. Now I am left with this other Liberated Borg (unassigned to a field) in my inventory. When I try to use it, it says "Can only use one type of this character."