You should be able to focus any targetable unit, pc, npc, creep, etc...

However I don't know that "focustarget" will be propagated outside of a party so you macro's may not work if you are using them for targeting.

The more I think about it the more I think this should work without problems... to test try this.

With warrior target an NPC.
With shaman target the warrior.
Type: /focus
Type: /target focustarget

You should have just assisted the warrior. If you end up on the same target as the warrior you are functional but your macro's are broken.