How to add code and html markup to another VS solution file without having access to it or subversion

A company hired me to add some functionality to a pre existing .NET site, the problem is I have to create it in my own VS solution file and somehow integrate it into the exi开发者_如何学Gosting site without access to the original VS solution file. The pre existing site is not in subversion. I coded up some stuff that works on my end, but they can't add it to the application with out getting some sort of server error. Is this possible?

Is the functionality such that they could create a virtual directory under their site, and put your work in that?

Beyond that...what's the server error they're getting?