Yes you may use them if there is no expiration date on the back, no they cannot be applied to new tickets, unless you want the ticket with the Magic Plus options. Then it can only be one ticket applied to one ticket. Honestly, you may be better off using them "as is", because the most they would be worth would be about $20.... which doesn't cover the cost of the Magic Plus option, if you would not otherwise be getting it.
As for using them, you may enter PI without a ticket, but if you want to go into the clubs, you will need to have your ticket to give them at the club door.