So basically you want your C# dll to populate a binary tree that you can then interrogate from VBA.
You will probably have to implement methods such as Load, First, Next, Count etc. and use them from VBA to get the data out of the DLL. I don't think you can use a C# proprietary binary tree for much in VBA, but I will probably stand corrected shortly.
You may get some hints from this answer.
installing and Refereing .net dll programmatically from vba app
You've basically got to create a COM dll. I've had various issues trying to do so with .NET in the past - and I've resorted to make my dll in Delphi, because it just worked without any of the hoops.