You can do it, but it is not cost effective unless you have tickets left over from a previous trip. Buying tickets from
UT won't cover the cost of the one-day ticket you have to buy from WDW. It would be cheaper to get your tickets through Disney in that case, unless you intend to hold onto the one day ticket and upgrade it for a future trip, or if you have old tickets for the rest of your days at the parks.
Using Disney prices, just for convenience (UT will be a few dollars cheaper):
one day ticket with package - 79.88 for adults
four day ticket that expires in 14 days - 233.24
five day ticket that expires in 14 days - 236.43
Here is the complete list from mousesavers.com:
http://www.mousesavers.com/wdwticketprices.html
UT will save you a few dollars over Disney's prices, but not the $80 you are paying for a ticket through your package. It's cheaper to upgrade that ticket to the number of days you want, or you can save it and upgrade it for a later trip.