It uses the same theme and plugin concepts from framework builder, so it should be relatively easy to update to use your look and feel and add links to your existing site.
The only complication would be using with your existing users table since the table names and column names would have to match up with the powermessenger solution.
You could probably import your users into the powermessenger users table and start using the powermessenger pages and then just update the templates to fit into your site.