Again another instance of needing root access!
It is possible with root access and the files on the usb stick are .wav mono, 16 bit, 8000Hz
Web GUI PBX tab -> PBX Configuration -> Music on Hold -> Add Music Category enter fromusb (or name of your choice)
save and apply
Insert usb stick goto Support tab choose messages from the system log files drop down menu
scroll to the bottom and look for anything about usb mass storage device mine happens to be sdb:sdb1 but only the sdb1 is needed
from the cmd line with root privileges enter mount /dev/sdb1 /var/lib/asterisk/mohmp3/fromusb
if it mounted you can enter ls-l /var/lib/asterisk/mohmp3/fromusb and a list of files on the stick will be displayed
Now choose fromusb as your MOH source on inbound routes etc.
Remember to unmount the stick before removing by entering umount /dev/sdb1 from the cmd line