I'm not sure whether you can add features that late (in October after using the pass in April) but it does not hurt to ask.
Next time, you almost could not go wrong by purchasing the plus pack before you returned home from a trip not consuming the whole non-expiring pass, if you went to Disney water parks later, twice anytime during your life, it would have been worth the money.
You could trade up the pass (only towards equal or more attributes which in your case means another 10 day non-expiring hopping pass). The gate price of your existing pass is $367., prorated for 7 days remaining gives $257. trade in value leaving $158. balance due since the new pass with plus pack costs $415.
Disney hints:
http://members.aol.com/ajaynejr/dispass.htm