You can use some of the Text fields in the MIGO Item details "where"-Tab , like " Unloading Point" " Goods recepient" or "Item Text" field. This data is stored in MSEG table.
One of my earlier clients had inserted a new Tab to capture additional data at the MIGO Item detail level, they used the BADI MB_MIGO_BADI.
Some more info on this BADI and implementation :
http://scn.sap.com/thread/948218