Take a look at the AGC->Tools Bulk Load "Insert User Account Attributes" and documented inthe KC:
https://www.ibm.com/support/knowledgecenter/SSGHJR_5.2.4/com.ibm.igi.doc/CrossIdeas_Topics/AGC/AGCore_Insert_User_Account_Attributes_Record_Track.html
You need the Account Configuration, IGI User (Master UID), the Account Name, and Default Account status. I believe, that this will match an unmatched account to a user. But test this out, on one or two accounts in a lower environment. I would normally try this out before posting, but my lab is down for a few days. So make sure this does not clobber any of the existing account attributes.
#Support#SupportMigration#Verify