It depends on if you have the regular Florida Resident Tickets or the FL Res Discover Disney Tickets. The former. its my understanding that yes you may add a day (however you won't be able to use it if the date you go to a park is a blocked out date). With the latter it is my understanding you can not add days onto it.
But keep in mind YMMV because this is only what I was told by a phone rep and a guest services rep and from what I gather from on here, they don't always have the correct information.