A zone can be programmed to be either wireless or hardwired. You could add another wireless motion detector to the existing wireless zone, otherwise you will need to remove a hardwired detector and change it to be a wireless zone.
First you will need to go into programming mode - press *8 5555 (or your installer code)
Then you will allow you to set the zone types. Type in 001 to go to section 001. Here you will enter the two digit zone types for zones 1 through 8. Go to your chosen zone.
Next you need to enter in the serial number for the wireless sensor. This should be on a sticker on the sensor and will be labeled as the ESN number.
Go to section 804 and enter your chosen zone, then type in the ESN number for the sensor. When you are done press #.
Finally you need to set the zone as wireless instead of hardwired. Go to section 101 and toggle 8 on (if 8 is not showing on the keypad then hit 8 to turn it on, then hit #, if 8 is showing then just hit #). Then go to section 102 and toggle 8 on to set zone 2 as wireless, go to section 103 and toggle 8 on to set zone 3 as wireless, etc.
When you are done hit # ***** exit programming.