Hi, Kori!
You can certainly purchase a one day ticket on your arrival day, but you cannot add it as your "7th" day to your package unfortunately. You will not receive your park hopper tickets until you check-in at Port Orleans French Quarter.
A one day ticket is a bit pricey, and since you already are visiting the parks for 6 days, why don't you try some other Disney adventures on your arrival day:
-
Rent a boat at Downtown Disney or at one of the resorts. We love the little 2 man Sea Raycers, so much fun!
-Spend the day at Downtown Disney. There is shopping, a huge movie complex, great restaurants, or even treat yourselves to a showing of
La Nouba!
-Go resort hopping! Take a bus to the Magic Kingdom then hop on the Resort Monorail. Stop at the Contemporary, Polynesian, and the Grand Floridian. Check out their shops, restaurants, or go for a walk on the beach.
-FORE! Golf anyone? There are several
golf courses on property to choose from. Or, if you "golf" like I do, go try
mini-golf!
-Ride 'em cowboy! There is even
horseback riding in Fort Wilderness!
With so much to see and do, I think you will find plenty to keep you busy on your arrival day in Walt Disney World. Enjoy!