Adding speaker terminal to Receiver?

My Receiver has A+B speaker terminals in which I have connected 4 satelite speakers but there times when I would like to use just one 15" 3-way speaker so I would like to add one more left and right terminal to connect just that one speaker, this speaker will NOT be used at the same time as as the A+B so what I really need is some sort of "switcher" I guess, this is neccesary only so I won't have to keep switching wires between one or two of the satelites and the 15", what can anyone recommend please? Thanks.