"with two expander modules to support up to 24 hardwired zones, of which 20 are currently used"
Adding the serial numbers in section 804 is correct, start with zone serial input on 21
Once all the new sensors serials were added enable your zones in panel programming sections 204 / 205 for enabling zones 21- 24 and 25-32
Program section 002 - starting at zone 16 to 32, remember to write down zones programming that was already programmed previously cause you must start with zone 16.
Go to location 121 (starting zone 21) to 132 and only enable option 8 for zone to be a wireless zone