EPiServer is built on top of asp.net, so it's just to add EPiServer functionality in your existing project. This is a good starting point: http://world.episerver.com/documentation/developer-guides/CMS/getting-started/

You will need to convert your existing aspx/views to Episervers pagetypes and blocks. Roughly you can say that an old aspx => new pagetype and an old usercontrol => block in Episerver. There are no automatic tools to do this but since both use .NET in the background, much of the code can probably be reused.