Yes you can do it. Cost depends on what ticket you have now and what ticket you are going to get. The transaction can be done at any Guest Relations location at the theme parks or at DTD.
The difference between a 7-day and an 8-day ticket is ~ $3 - and that's with a hopper! If you want it to not expire, that will be an extra $55 plus tax for the 7-day and $100 plus tax for the 8-day.
The only way to get these in advance at the current time is by calling Disney. Eventually they will make their way to the Disney web site and to the ticket wholesalers who elect to carry them but right now, it's from Disney by phone only.
We currently have 7 day park hopper tickets for our upcoming trip. So, if I understand this right, when we check into our hotel and get our tickets, etc, we can upgrade our tickets for 8 days for only $3 per ticket?
Yes, $3.00 to expand a 7 day MYW to 8 days if it is unused or has been used for less than 7 calendar days.
Currently they take the gate price of what you have and the gate price of what you would like to have and you pay the difference. (No change returned and no removal of attributes).
I did this upgrade (via the Disney phone line) shortly after we got our 7 day tickets. Disney made the 8 day ones available so I called (thinking it was going to cost at least $1,000,000 to add a day on). I was SO surprised when the CM said the total cost to add 1 extra day for 3 people with PH was $8.51 TOTAL! I was so excited I thought I was going to . . . well, lets just say I had to run to the restroom! It was AWESOME!!!