From the msdn below.
Developing COM Add-Ins for Microsoft Office 2000
Office 2000 should support IDTExtensibility2 Interface.
Also we can consider PIA as kind of special com interop assembly. For
office xp and 2003, we have provide the downloadable pia in our website.
But for office 2000 we did not provide such a pia, but we can do it
Primary Interop Assemblies
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