Well, this is the solution I came up with. I don't like it, but as far as I can see, it's the only way to merge attributes without creating Identities.

1) Query the igops database with the following query.
SELECT USER_ID FROM SUSER_ALL_V WHERE effective=TRUE AND deleted=FALSE;
2) With the results of the previous sql query, query the AD and get the DN.
3) Export the returning DNs to a CSV File and consume it as an identity in IG.

A tad too convuluted for my taste but it might help someone in my situation.