Yes, it’s possible to only backup select repositories.
BackHub creates backups of all repositories it gets access to by default. If you want to backup only select repositories you can simply limit access to those repositories you want to backup.
In the installation dialog you are being asked to choose whether you want to give access to all or only select repositories. Please choose “select repositories” and type in the names of the repositories you want to backup.
If you have already installed BackHub, you can edit the permissions by managing your installations on GitHub.