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.
is there any chance to migrate the .Net project into EpiServerCMS, if yes please help about this.