The JobRoller theme is not a plugin, so it can't be installed on an existing theme in that way, however you could install JobRoller in a subdirectory,
e.g.
yourdomain.com <== Your existing homepage
yourdomain.com/jobs <== Your JobRoller installation
No coding would be required in this instance with the exception of sharing the user database (only if required), but that is WordPress specific, not theme specific. Thanks.