6FL USB audio interface is under development for X1. But even if there is the retrofit kit, it will be very expensive since the TCU unit has to be purchased separately. If you have the bluetooth phone interface already it will be much more cheaper since the TCU is factory installed with the bt option. You can have a look at the Dension Gateway 500. Dension enables both using a USB Stick and iPod.Web page doesn't say anything about X1 compatibility but I think it will work.
http://www.incarmusic.co.uk/product/...-iDrive-System
I'm also attaching the current X1 accessories list. Hope it helps.
http://rapidshare.com/files/43305224..._prices_x1.pdf