is there a way to add an existing classic ASP webapp into a solution in VS? The application is around 4000 files large and currently maintained outisde Visual Studio.
You can't do this with Visual Studio 2008 out of the box, but you can if you install Service Pack 1 - see Scott Guthrie's blog post for more info.
Edit: To clarify, whilst you can create a project out of the box, there is no intellisense or debugging unless you install SP1
The best way to do this IMO is:-
In VS 2008 File | Open | Web Site...
In the dialog ensure File System is selected. Navigate to the physical folder that represents the root of your Web app.
Click Open. Job Done.
There's no real point though is there? As Classic ASP files aren't compiled. I keep just opening them individually as needed from File Explorer.
Although you could add them all to a VBProj or CSProj so you get them all showing up in your Solution Explorer, and could use features like Project -> Deploy.
