You can read more about customizing our themes, including creating a child theme from our
developer center documentation here, with more specific information about child themes from the
WordPress codex here. Yes you could certainly prevent posting of jobs from the front-end and post only via the
wp-admin if you wanted to, simply by removing the ability to register on the site.
In order to migrate a large amount of resume data you would likely need some knowledge of mysql admin, as there is no specific tool within JobRoller which will import resumes. WordPress itself has an import/export tool, however this would still require some additional database side changes to ensure resumes are migrated to the correct location.
You may wish to note that we do have a
certified partners page where you can find developers for hire who are experienced at working with our products. If you make contact directly with an outline of your requirements, they can provide a quote for the work. Thanks.