When adding a new keypad you'll need to make sure you're not assigning it to an occupied address, to make things simple I'm going to suggest adding your new keypad to address 23 (keypad 8), that way you're unlikely to cause a conflict with any of your existing keypads. You can use any existing keypad to enable additional keypads.

Enter the current installer code
(factory 4112) then 8 0 0
If successful a custom alpha keypad will display "Installer Code 20", a fixed alpha keypad will have a flashing 20 displayed.

Click "Next" to continue...