Excel add-in in C

What is the best way to add functionality to an open Excel spreadsheet? What I want to is to read the contents of an open spreadsheet, manipulate the data and place it back into the Excel spreadsheet cells. I'd like to write a simple program in C to do this. Is this possible? My alternative is to have the user save the file, read in the contents and then write the file back out, but I suspect there is an easier way . I'm a casual programmer(read engineer), so the simplest soultion wins.

Thanks, Brad