If you are referring to Numbers, part of iWork for Mac computers, I am not
aware of such an add-in.
The usual workaround is to use Numbers to save the file in a format that
Excel can open.
The web page http://www.apple.com/iwork/numbers/ says "You can open
Microsoft Excel files in Numbers or save your Numbers spreadsheets as Excel
If you have other Mac-related questions, I suggest posting on the very
active newsgroup microsoft.public.mac.office.excel.
Just wondering if there is an add-in that will let Excel open a numbers file,