If you are a journeyman for a merit-shop then it is a little complicated.
There are a few different ways to join the union.
If you have 10 + verifiable years, I believe you can be "grandfather-ed" in.
If you have 5+ verifiable years, they will have you test and place you accoringly in their system.
ie: If you do well on the test, you will turn-out at JW, if you do poorly on the test you will be placed accordingly in the IJ (Intermediate JW) program.
If you have less than 5 years experience, more than likely you would have to go throught the apprenticeship, but special arrangements have been made for people before.
Pay ranges anywhere from JW scale to lower pay for IJ's to even lower for Apprentices.
It would really depend on where you were placed.
This is the way it is in GA, I do not know if this holds across the whole U.S.
Hopefully that answers some of your question.