Can I upload a specific employee number so that it appears in the document I'm sending?

What I'm looking for is similiar to a mail merge. The end result I'm looking for is to have an individual number attached to individuals who have been uploaded as a group. So I could upload a spreadsheet with their email address and employee number and the end result is that each person receives an email with their unique number. I have worked with the "upload" contacts where you get the columns to match your spreadsheet, but after I do that, I'm not really certain where that information lives or how/where it shows up. Or how to get it to fill in a field in an attached document. Much thanks for any advice!