We also wanted to do a ten night package. The only way to do it is to combine two packages. Each package must contain park admission, so in the end you will pay a bit more for admission (but if you are doing a dining plan you may save it there). The park tickets do not have to correspond to your dates of stay however dining does. So if you only booked a package for the first 8 nights of your stay you would not be allowed to use any of your dining allowances on the ninth night. You could however still purchase the more cost effective 10 day park tickets with an eight night package. We are splitting resorts, staying 10 nights, and want dining so the most cost effective way (with a package) is to book 1 package for our first 5 nights at BC, with a 9 day ticket and dining. Then a second package, for our 5 nights at AKL, with a 1 day ticket and dining. We would really like to do the dining plan but haven't yet decided if it will work for us to do 2 packages. For one because we will end up paying about $125 (total) extra just for park admission, then we would just buying a single 10 day ticket for each of us. And two because of now we'd have to forfit our room discount (about $458 so far) to get the packages. So we will only do it if a code or pin comes out that allows us to apply a room discount to a package AND if that discount is good enough that we feel we can eat the extra $100+ in park admission and still come out ahead (on the dining side).
You can book longer by calling because they mesh two packages together. We did the numbers and it's expensive, mainly because an 8 night package only costs a little more than say a 3 night package. It's a big pain and quite frankly, I think it's stupid. However, I guess it all worked out because we bought AP's and the DDE...so we have many discounts coming. 
