If you want to do this, it would probably be better to upgrade it to 10 days and add the Water Parks & More option. Then you would be able to use the remaining water park entries for trips in the future for a long time. If you only ever go to the Water Parks once during a trip, then this can be a better option than adding the WP&M option to every set of tickets for every trip, or paying for the individual water park entries.
10-Day Magic Your Way Ticket w/Water Park Fun & More & No Expiration 10+ = $608.12 (includes 10 water park & more entries)
Water Park x 10 = $553.80
Keep in mind that you can also purchase 6 day tickets and add extra days when you know you're going to need them (within 14 days of first use) instead of going with the 8 day from the beginning.
Adding no expiration almost doubles the cost of your tickets.
8 day base ticket = $292.88
8 day base ticket + No expiration = $500.55
6 day base ticket = $275.84 (adding an extra day only costs $9 or less)
6 day base ticket + 2 day base ticket (on a later trip) = $454.76
Even if you consider the fact that Disney prices go up every year, you would have to plan to spend only 2 days in the parks on a future trip and you'll probably end up spending more to add the no expiration option now. You would also have to hold onto the tickets until you finished using them.
Once you get outside your 14 day window, you can't add anymore entitlements to tickets, even if you added the No Expiration option previously. So if you do choose to add it to the 8 day ticket after you've already used 6 days, you will not be able to add days during a future trip.
(If your tickets will include Park Hoppers, the above calculations will be wrong - information gotten from
http://www.mousesavers.com/mywpricechart061211.html)