Well, I don't know how you could do it dynamically, but you can always build your own dictionary using the "create" option to aspell. Here's the official site to look at for details:
http://savannah.gnu.org/download/asp...00000000000000.
The only real problem I see is that there is no way to add an individual word; you have to create a whole new dictionary based on some "master" dictionary and a user-defined word-list. I suppose you could just keep copy of the plain-text word-list around and rebuild the dictionary every few days using the official English dictionary as your base.
--Rob