Generally, ring-down lines (Stations that can only call one destination, and it's dialed when the line is picked up) are created from analog lines. It's kind of a waste to put in a multi line station for a ring-down.
If you're going to put a button on a multi line station that only calls one destination, it would be simpler to use an autodial button.
However, you might try removing all of the call appearances of the 2420, and putting in a bridged appearance of an analog station that is configured as a ring-down. You'd put in a "hot line" destination of one of the abbreviated dialing list members.
Carpe dialem! (Seize the line!)