You may only "add" three more days to what was once a 7 day pass, this being done within 13 days after first usage.
I believe you can "trade up" the partially used pass before 14 days have passed. With one out of the 7 park days remaining the trade up value towards an MYW pass is 1/7'th the original gate price.
If the original pass was expiring, it is not worthwhile to add non-expiration without adding the 8'th, 9'th AND 10'th days unless there are at least 2 plusses remaining as well and furthermore, it is then not desirable to trade up on a future visit because the plusses will be lost.
Disney hints:
http://members.aol.com/ajaynejr/dispass.htm