The number of posts which displays on the homepage, and syndication feed numbers is determined by you in the WordPress admin dashboard "reading" settings
here.
JobRoller doesn't have a built in importing tool, so it would likely require some knowledge of mysql admin in order to migrate jobs over to the correct location from your existing theme (i.e. JobRoller uses custom post types, which many other themes do not). This is essentially outside of the scope of what is officially supported, however we do have a
certified partners page where you can find developers for hire if you're looking for someone to assist with this work. If you make contact directly with an outline of your requirements then they can provide a quote for the work. In a related note, there is an
XML job feed generator plugin available in the AppThemes marketplace for JobRoller, however this is designed specifically for Indeed - it may give you an idea of what can be done though. Thanks.