You are correct, there is no .csv importer built into JobRoller, so it will rely on your knowledge of mysql admin in order to migrate this existing data into JobRoller. I found a thread related to this within our forums which had some tips from the developer which may give you some idea of what is involed:
1. JobRoller uses it's own tables only for storing job packs, job alerts and orders.
2. The first and main step would be to import all your posts and users to WP tables ('wp_posts' and 'wp_users').
3. After having this data imported, you will need to update the post type of your posts to 'job_listing' using an MySQL Update..
4. From here you should be able to update all the additional data directly from the backend or frontend (custom fields, user roles, etc...).
Hopefully this helps. Thanks.
