Currently it's not possible to create/specify plan branches for existing branches (in the repo) using Specs. From the Bamboo Java Specs Reference documentation:

Bamboo Specs does not have an option to manually specify plan branches at the moment. You would have to use Bamboo UI to manually create branches after plan is created.

This has to be done from the Bamboo UI. It would be nice to have the ability to manually specify plan branches using Specs.