I have a bunch of applications that run on the free tier and are barely used (they serve websites that are rarely used).
I also had an application that was used 24/7 and used up all my free tier hours. I've bumped that to a Hobby dyno by now, but now all of my free tier applications aren't working.
Can I purchase some more free tier hours? Otherwise, the only way to make all of those sites that are rarely used to work is to make them all hobby, which isn't worth my money.
The answer is no. You cannot buy free tier hours. Your Heroku account has a total amount of 'free hours' per month, and if you use them all up they are gone.
You can upgrade just the applications you care about to the Hobby plan.