For Magic Your Way passes, hopping costs $35. regardless of number of days on the pass, regarless of the days remaining and regardless of how much you paid for the pass if you add it on or before the 13'th day after day of first usage.
After the first 14 day span of usage the remains of the (non-expiring) pass can be traded in towards a new pass with hopping and perhaps more days. This will cost you somewhat more than $35.
Can be done at Guest Relations just outside the turnstiles of the second park you are planning to visit and for which you needed hopping.
Disney hints:
http://members.aol.com/ajaynejr/disney.htm
Certain promotional passes (non-standard; unlike anything sold at the gate) cannot have anything added.