It is possible to write a macro to do this, but a simpler solution is
to open the spreadsheet you will be sending them then go to
Tools|Customize... and on the Toolbars tab click New...
This will make a custom toolbar. Drag your custom menu on to this
toolbar. Then, click on Attatch... and select the custom toolbar you
just made. Then click on Copy then OK then Close.
Your custom toolbar is now stored in the spreadsheet. When the person
you're sending it to opens it, they will have your custom toolbar, with
the menu on it.