The Title, Description, Tags, etc. for each image are input to cells in a spreadsheet by a group of people who are not very technically literate. They can manage this, but would probably not be able to handle metadata, so I believe the spreadsheet has to be the initial input document.
Perhaps it is outside the scope of Zenphoto, but I feel it would be a useful enhancement for crowdsourced projects involving a large number of images (I have around 80,000) if input from spreadsheet was possible.
Perhaps it is possible to have an intermediate stage by transferring data in bulk from a spreadsheet into metadata?