There is no built-in way to have that checkbox automatically checked. However, a code modification was made for that version of UBB that would make it automatically checked. Of course, you would have to be able to talk the board owner into editing the script files.
If they are interested, they can find that signature modification by
clicking here. I would also note that signatures are default to on in UBB v6, so they may even want to consider upgrading to the latest UBB.