Project Architecture - Adding new user control to existing web application

[Amil]

I have a reusable web application (e.g. portal). I often do a project copy
to create new virtual apps. This works fine.

Now I want to take that reusable app (not the solution and it's projects)
and just add a few new user controls to it. I do not want to copy the
entire reusable application (solution, projects, etc) and add my new user
controls.

What is the best practice for taking an existing web application directory
structure, and creating a new VS solution/project so that it works inside
the reusable application? I want to create a new VS solution/project that
is not a web application itself, but SOMETHING I CAN CREATE USER CONTROLS
(.ascx files) in it. What type of project can I create to hold .ascx files?

Amil