Yes, you can add a day on. Most of the time its about $10 to add a day on to a 5 day hopper. If you booked thru Disney travel (including AAA and costco) you can take your ticket to the travel office in DTD. They are open daily until 5. If you ordered your ticket from the website I think you'll take it to the ticket booth.
You can add a day on the last day, as long as you do it before the travel office/ ticket booths close. You just can't come back the following day and add a day.