Hi,
Unfortunately the short answer is "no".
The Office Add-Ins are built on Microsoft's Visual Studio Tools for Office code, which allow developers to create managed code add-ins for Office 2003 and 2007. The interface into the Office 2000 and XP suite is via COM, which would require a rewrite. Also, Office 2000 is getting pretty old now and, I believe, is unsupported even by Microsoft.
On a more positive note, the web scripts which provide the add-in user interface can be reused should someone take it upon themselves to do the C++/ATL interface work required.
Thanks,
Mike