I have a user that does not have Credentials/* permission at root level. This user has all the Job/* and Credentials/* permission under a specific folder.
In the job configuration, this user is not able to add credentials as the Add button is disabled.
Example shows with Subversion, but same happens for other plugins (Git, Mercurial, SSH Agent, ...)
With these permission, this user is able to browse to the Folder Credentials store and add credentials.