24AWG is fairly common to power older hardwired panel keypads. There should be at least four conductors running to the keypad location.
You can of course re-use the cable for the new plug-in transformer, but in this case you would want to double up on the conductors. If four are available, use two for positive, two for negative. This increases the effective size of the cable.
Earlier model Qolsys panels had a notably low tolerance for voltage drop, but IIRC this was addressed in newer panels.
Always use the included plug in transformer to power the panel.