Disney allows you to add on any of the available ticket options to an already purchased or even a used MYW ticket. The add ons available are the Park Hopper option allowing visiting multiple parks on the same day, the Water Park Fun & More option which gives a varying number of visits to the water parks, DQ, PI, or WWoS, and the no expiration option which will make all your unused admissions, both theme park and WPF&M options, never expire.
These add ons are done on a fixed cost basis. That means if you have a 7 day MYW Base ticket and you decide to add the park hopper add on for the last three admissions remaining on the ticket, you will pay the full $45.00 + tax to do it, not a prorated amount. This holds true for all of the add ons including no expiration. They are all a fixed price based on the original length of the MYW ticket, not what's left unused when you do the transaction. The price is never prorated.
The no expiration option must be added within 14 days of the first use of the ticket. You can even have used park admissions before doing the add on. The add ons can be done at any Guest Relations location at the theme parks or at DTD as well as at any Lobby Concierge location at a Disney resort. Note that Lobby Concierge cannot do any transactions involving AP/PAP/SP's. The Park Hopper and Water Parks Fun & More options can be added at any time within the 14 day window.
Thanks for confirming that the "plusses" don't expire- I was pretty sure that was right, but since these guys usually know what they are talking about, I wanted to check in and see. 
Me too!!
