You don't really "add" the Webserver to an existing app Thys, because your existing app is a desktop app, and a WebServer, is, well, a server.
So you'd want to start with a new app, perhaps using your existing dlls if you have them.
Your user will start and stop the desktop app as normal, but the Server runs 24x7 in the background, usually as a Windows Service.
Incidentally ABC is better than Legacy for the Web Server because it has better file handling. Legacy struggles with things like auto-numbering. since it's a new app though it can be based on ABC. Pretty much everythingin it is not ABC or Legacy anyway - it's NetTalk procedure templates.
cheers
Bruce