I'm sorry, but you don't have any extra time. It appears that you arrive in Barcelona on March 31. That day is pretty much lost to jetlag and the exhaustion that comes from a (usually) nearly-sleepless night sitting up on a plane. (Perhaps you've traveled to Europe before and know that overnight flights don't affect you so badly??)
Your one full day in Barcelona is April 1. The next day you're off to Seville. The train time alone will be over 5 hours; that doesn't include the time needed to pack, check out of your hotel, get to the train station and repeat the process on the other end of the trip. Effectively, April 2 is pretty much shot--just a travel day except for some casual strolling around Seville and dinner, or tapas if you don't want to wait to eat until 10 PM.
You really need to apply most if not all of your post-Seville time to Barcelona, returning there on April 6 (better) or April 7. You shouldn't try to return to Barcelona on April 9 [corrected] if that's the day you're flying back to the US.
Morocco is absolutely fascinating, but I certainly wouldn't fly there for just 2 or 3 days. That's not a good ratio of sightseeing time to travel time. Barcelona/Seville's not a bad combination at all. I suggest sticking with it unless you want to drop Seville and substitute Madrid or Toledo.
