I'll assume that this hunt group is a hunt group of keys.
Let's call the keys 111,222,333,444
You want the hunt group to be 555.
A hunt group cannot have a MWI button. So here's the work around:
Renumber the key 111 to be 555.
Create hunt group 111. Put all members, including 555, in the hunt group.
Forward 555 via Rerouting forms to hunt group 111 on a busy and RNA to VM
Now you can put a MWI button on phones.
Ralph