I want to add an Embedded Data field to already existing contacts. How can I do this?

One way to do this is to download the data file that includes all the contacts. Once you've downloaded the file, you add a column with the title of the Embedded Data field you want to the second to last row in your doc. Give the contacts the information you want to add to that field. You'd then save the file, re-upload it to Target Audience (by selecting import in the Add Contacts tab) and you should be set.