I am replacing my garage doors and the new door set has all the necessary hardware except the rear hangers. This is fine as I have punched angle iron I can use. The problem is that the top bar does not reach where the old doors did which is where a joist currently sits. Because of this, my top bar does not reach a joist, so I don't have a place to hang my rear hanger. What can be done about this, if anything?
You do not need to add an additional ceiling joist to simply support the new garage door opener. Instead install some blocking between the existing ceiling joists similar to how it shows in the following picture. It may even be convenient to install two blocks to pickup both sides of the garage door opener top bar.
(picture image from build-my-own-home.com)
The blocking can be nailed or screwed in place through the two ceiling joists. Once in place the brackets or mounting bar for the door opener can be attached to the blocking.
Note that if the spacing between your existing ceiling joists is more than the nominal 16" or 24" then make use of a wider piece of material for the blocking to maintain it's rigidness even up to and including a piece of 2x12 material.