I've moved this to off topic as it is not a ULib/ULX question as much as it is a DarkRP.
That being said, I know there are customchecks in DarkRP, but I don't know if they are 'just' that, a check to make sure you have access to something before a job change is allowed, OR, if they could be used to set off a function once someone changes to a job.
I think customchecks expect a result though, true or false, before allowing someone to change into a job.
I'd recommend searching the DarkRP wiki or forum, instead of the Ulysses one.