Moving your existing site to DNN generally means identifying three items that need to be migrated:

1. Content - Moving content from one CMS to another generally involves doing a lot of copy/paste.  There is no straightforward migration path unfortunately.
2. Design - While much of the HTML, CSS and images are easy to move, you will still need to spend some time converting the theme from your existing site into the format required by DNN.
3. Functionality - This is probably the toughest of the three components to move. DNN has thousands of modules (both free and commercial) that are available so you might be able to find something that is functionally equivalent to anything you currently have.  If you had a lot of custom development, then you might need to do some redevelopment on DNN depending on how complex the problem is.