I wouldn't recommend it as it would be messy and could be problematic, I guess it depends on hoe many users you are talking about and the call volumes. You could setup virtual users on the IPO for the users s on the other system and then twin the virtual users to the other system users.
Virtual users need to be setup without VM and extend the no answer timer, not that either of these should come into play with a hunt group call.
You would also need to be careful of any diverts or no answer timers on the destination extension as they could process the hunt group call were a native extension wouldn't.
Also you may need to use (depending on the version of the IPO) forward not twining. I have an unresolve bug where logout out users wont twin hunt group calls even when Twin Hunt Group and twin logged out is enabled.
Make sure on the virtual user you enable twinning/forward on hunt group calls and also twined when logged out if using twining.