Probably the easiest way to assign a subscriber to a COS with administrator rights would be to edit the a subscriber record in the UnityDB SQL database.
From SQL Enterprise Manager expand the UnityDb and find the Cos table.
Right click on the Cos table and choose Return all rows. This will open the table. In the CosObjectId column, copy the CosObjectId for the defaultadministrator COS and close the table.
Right click on the Subscriber table and choose Return all rows. Find the subscriber you wish to assign to the defaultadminstrator COS and in the CosObjectId column, paste in the CosObjectId you previously copied.
Close the table.
You should be good to go.
HTH