You would probably be better connecting a unit to a co port if you can then you should be able to dial access a code to enable the doorstrike.
Alternatively if you leave the DPIM in place and assign a DSS key to it,they could use the open function with what they have.Little bit messy but it will work.

I had a customer with an inside lobby doorbox and I hooked up a spare digital set with a ringdown to a group and they can talk to the door but if they want to allow access they have to press the DPIM DSS key to open the door.It works