Talked to the BMW service dept in Hong Kong - 644 can be retrofitted but not 6NF, at least not now.
For more information on all these codes (6NF, 6FL, and 644), see this link:
http://www.bmw.com/com/en/newvehicle...on_0909_en.pdf
I don't have too many song, so a viable option for me seems to be getting the 644 and then copy all of my mp3s to the idrive's internal harddisk.