Just put the emergency extension into a calling group (use group 2, which is Intercom 72). Then either have your callers dial 72 from the auto attendant, or use a selector code to point to 72. If you have them dial it themsleves, be sure to set selector code 7 to Direct Extension Transfer (first digit of an extension number), because it is Not Used by default.

Calling Group: #502-G-XX-1, where G is the Group Number (2), XX is the extension number to assign to the group.