Creating a C# com addin that can call UDFs

[ep]

I'm relatively new to office development, and I'm trying to create an add-in
in C# using only visual studio 2005 professional without VSTO. I'm able to
successfully create add-ins using the Extensibility\Shared Add-In template,
but I need to create an add-in that both creates a new toolbar and calls
UDFs.

I've seen this done with vba wrappers or by loading the add-in as both a COM
and automation add-in, but I was wondering if there's a way to do this
programmatically in the code for the add-in itself. If this can't be done
with the Shared Add-In template, can VSTO do something like it?

thanks