I need another joist in my garage
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.