I have a similar situation, but we needed ACD agents to be reachable while on ACD calls. We used the agent id as their primary DN (x501) in skill groups, then made single line keys (x5501) on each agent that just has a "5" in front of their agent id. The agents/supervisors use the 5xxx extensions to call each other. I realize you are looking for Multi keys, so maybe someone has a workaround to get you there.