JobRoller (like our other themes), is a stand alone theme, not a plugin, so it's not designed to be integrated with an existing theme in that it will literally replace whatever WordPress theme you may already be using.
To use JobRoller with an existing website, you have the option to install it in a subdirectory on its own instance of WordPress:
i.e.
www.yourdomain.com <== Your existing website/homepage
www.yourdomain.com/jobs <== Your JobRoller installation
You could then simply link the two between their navigation menus..even more easily done using the WordPress custom menus function :)