If your only using Ethernet or Fast Ethernet (ie, no GigE) then you can. With Fast Ethernet only wires 1 (orange/white), 2 (orange), 3 (green/white) & 6 (green) are used leaving the others free.
It isn't worth the bother of knocking up the cables you'd need at each end though, I've done it in past but only when running a cable wasn't an option.