Esentially you're looking to do .NET interop on your "legacy" code. If it's not managed .NET, you're looking for a way to make BizTalk see it that way. How are you planning on using the code? Is this something complex that could use an adapter, or something simple that could be rewritten? Or somewhere in between?
I've found many ways to avoid calling libraries directly, and even created simple custom functoids with little effort vs. calling a library from an Orchestration. But, I'd want to know more about your plans before I made a serious suggestion.