No we do not have plugin versions of our themes available at this time, they are all stand alone themes. For an existing site we can suggest that the themes be installed in subdirectories e.g:
yourdomain.com <== Your existing site (essentially becomes the homepage)
yourdomain.com/classifieds <== ClassiPress installation
yourdomain.com/jobs <== JobRoller installation
And with additional styling of the themes to "match", you can achieve a unified site. Thanks.