AT90USBKEY as USB host
According to the user guide, p9:


  When connected to a mini B plug, the AT90USB operates as an “USB device” (the ID pin of the plug is unconnected) and when connected to a mini A plug, the AT90USB operates as a “USB host” (the ID pin of the A plug is tied to ground).


So yes, the board can operate in host mode so long as you use a USB-OTG cable to ensure correct setting of the ID pin.