you cannot integrate DNN in your website with reasonable effort, but you can migrate your content into modules on a DNN web site. DNN is not a tool, it is a framework.

Yeah I can see why you would want to use DNN for just the functionality you desire and avoid the work of implementing the framework since you already have the sites up and running.

But DNN has an extensive membership system that allows for unlimited roles, granular permissions and unlimited portals. In addition it has a Module API that allows you to inject custom code that can run differently based on it's current "instance". A module can have different permissions and visibility between different instances of it that are even placed on the same page.

Powerful stuff. Stuff you think you may no need right now but most live sites really do need this stuff. For example. where do you put the administration functions for your site?. With DNN you just put these users in the Admin role and they can only get to and see the admin pages. If certain users have different abilities you just assign them to the correct role and configure the pages Seems like  a small thing unless you have to code it manually.