You can only schedule FP+ for the number of days you have tickets. If you have a 5 day ticket currently, you can only book 5 days worth of FP+.
I'm in the same boat. My tickets on my package are only 7 days but we've decided to be in the parks 8 days now. I picked the one day that I thought FP+ was the least critical for and didn't schedule FP+ for that day. It's one of the later days in our trip and we're only spending a few hours at AK and will be hitting it at rope drop. I'll upgrade my package tickets after I use them the first day and schedule FP+ for the remaining day at that time.