You can't really do an RJ-31X on a wireless jack. It's designed to interupt the path between the external telephone line and the internal phone lines.

What you can do is connect the alarm to the wireless jack and forgo it's ability to cut out the internal phone lines. While the plug is LARGER for the RJ-31X, the pair for the outgoing phone wire is the same. If the alarm has a socket on it's end, you can just connect a regular phone line between it and the wireless jack (it's designed to plug into the center). If it has a plug, you can get either an RJ31X or another 8 position modular jack and connect the center two wires (terminals 4 and 5) to the red/green (center) wires on the wireless jack.