(My comment to Sergey's post as an answer)
You can expose .NET to COM operations just like they were ActiveX objects, see:
Exposing .NET To COM (pay attention to the very last answer)
MSDN .NET COM Operations
Another good tutorial
But be aware, the transition from a "large vb6" library to .NET will be very painful, you will have to rewrite a lot of it. Try not to do a 1->1 conversion, .NET has some features that weren't available in VB6 that may make your life a lot easier.