It's true, like @cdenesha mentions that these are separate sections of the Jamf Pro server and aren't connected, but that doesn't mean the User EA values can't be updated via the Jamf API. The /JSSResource/users section is available for updating in the API, so this should be possible, even though I've never done it myself personally.
I can't think of a simple way to do this in bulk, but that shouldn't really matter. If you had the script run as part of the initial policies that kick off on a device after first login by the user, it could work. There's also a danger of running into more than one user with the same name, but I don't know how detailed the user names are in your setup. You mentioned they come from AD, so it should be ok since they should all be unique.
As I said, I've never done this myself since I don't really use the Users section in the server, but if you need a little assistance in getting a framework together for such a script to test it out, post back and I'll see if I can make some time to put something like it together.
