If you boot VirtualBox from the ubermix install CD iso (
http://wiki.ubermix.org/page/CD_Install), you can install directly from a customized USB key. Download the iso from the page above and setup your new virtual machine (Ubuntu Linux) to boot from the CD. When prompted to insert an ubermix key, insert your customized key and click on the little USB icon at the bottom of the VirtualBox window to select it. Lastly press any key and you should be greeted with the familiar install menus.
When complete, you'll also want to install the VirtualBox guest additions, so boot the VM in generic mode and choose VBoxGuestAdditions.iso from the CD menu. This won't actually mount the CD, unfortunately, so next you need to open the Home folder under Files and click once on the "VBOX..." CD icon in the left sidebar. Once you see the files on the CD, open a terminal (ctrl-alt-t) and type "sudo /media/VB[tab]/VBoxLinux[tab]" (pressing tab auto completes) to install the drivers. When complete, reboot, and you're good to go.