I am going away, camping, and will have no access to Duolingo for a week. Can I 'buy' 7 days' worth of streak-freeze so that I don't lose my streak?
You can't. I wish they added it, it's useful when you can't use Duo, for example because you have no Internet access during your holidays :/
If you have absolutely zero Internet, there's nothing you can do, but if you can manage to turn on the data transfer just for a moment every day, you can just buy another freeze streak every day.