Classipress is a wordpress theme and so it would replace your existing theme. If you want your existing theme setup to remain you can just install another instance of Wordpress-Classipress on a subdomain. You can then create a menu link on your main site that points to the classifieds install.
Please help our moderating team work more efficiently by not sending us support questions via PM. You can read more about how AppThemes support works
here. However, you can send a PM to follow up and remind me if I missed your support request/thread.
Thank you and have a nice day.
