How can I re-program the outside line buttons a Merlin 410 phone system?

I have the AT&T/Lucent/Avaya Merlin 410 phone system at my office. The top left 4 buttons are pre-programmed for the outside lines.
Since we only have 2 outside lines, I would like to re-program the other buttons to one of the available programs so that I do not have 2 "dead" buttons.

I tried following the manual, but it is not holding the program. I know I am doing correctly because it is holding on the 2nd group of 5 buttons (I am using the 10-button receiver).

Can anyone tell me if this is possible; if so, how?

Thank you in advance!

-Matt