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?

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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?