How to Copy a Standalone Install to a Multisite Location

I have a website development business and we have created a wide variety of website for different customers. When we finish a site, it looks nice, but many times the clients go in after the fact and add crappy images, etc that make the site looks less than stellar.

I would like to take these sites right after final creation and copy them to a multisite install that is a collection of the sites we have created -- originally -- without the client's post-launch edits. This way, I have a comprehensive collection of interactive websites (not just screenshots) that show the sites we've created without worrying about what the site may look like today.

How can I copy a freestanding installation [URL.com] to an installation inside my portfolio collection [portfolio.com/URL]?

Thanks in advance for your help.

Jeff