I know this was asked almost forever ago, but it has a lot of views so maybe my answer will help someone. I am looking into this at the moment and I don't think that the imdisk solution will work because imdisk is the software used to mount the drive. VirtualBox doesn't have any knowledge of how to handle an 'ima' file.
If you try mounting a real USB drive, you can see that the USB drive is unmounted from the Windows Explorer shell and then the USB device is assigned to the virtual machine. In Virtualization terms, this is called 'passthrough'. The reason why this works is you are giving the guest direct access (for the most part) to the hardware and the guest OS knows how to use it.
The best that you could do in this case is probably set up a virtualbox shared folder in the mounted 'ima' container. In this case, you could use it for storage, but you wouldn't be able to set it up as a bootable device for your guest.