In Excel, how can I add a popup definition to each column header in the header row?

I’m using Microsoft Office 2007 Professional, and Windows 7 Enterprise Edition, so I can save the spreadsheet as an .XLSX file; but I’d prefer to keep it an .XLS file. That’s because I need to send this file as a database template to customers who have all sorts of Operating Systems and Spreadsheet Programs. Therefore, I want to make it as universally compatible as possible.

The template is for database uploads to a Yahoo! Store. There are a long series of variables in each row of the database. I need to define each, tell new Yahoo! Store users what each is and where it ends up on the item or category page it generates, tell them what limits and data standards apply to the given variable (column header), and explain how best to write content for SEO.

However I do this, the data can’t export when you save the file as a .CSV file. Excel experts, can this be done? If not, maybe I need to make a video explaining the database template. I have a Word document file where I wrote up all this info, but most Americans are so resistant to reading that they invariably call me up and use up 2 hours of my time asking all the things that are in the Word .DOC file I already sent them.