Merging two worksheets and setting a migration date

I have gotten a LOT of mileage out of the help that was given earlier, and I have been able to apply it to a number of different scenarios, just as I had hoped and expected, so THANK YOU VERY MUCH!

I have one more twist to the merge case: this time, I have a list of accounts that I want to migrate to a new system, and I have a feed of the Active Directory users that currently exist. I have already figured out how to identify those entries in the list of Accounts to Migrate, and also to create a merged list of both the Accounts to Migrate and the Active Directory users.

The one remaining thing I have left to do - and maybe it's just because it is late in the day and I have a "brain freeze", but what I want to do is take all data in the Active Directory that is NOT included in the Accounts to Migrate and add all those Active Directory rows not already in the Accounts to Migrate. I have all the information I need, and I have the Active Directory in spreadsheet form.

How do I identify Active Directory rows not in the Accounts to Migrate and add each row to the Accounts to Migrate that is in the Active Directory and not in the Accounts to Migrate without duplication or error?

Attached is a contrived example for use in this discussion.