Yes you can.
For now we have not standardized this feature yet, but you can achieve it by following this procedure:
- 1. activate DEV mode as described here
2. make a new directory under /mnt/MPD/USB just like /mnt/MPD/USB/SDCARD
3. put your files in the new directory
4. go to /sources page and force MPD database rebuilding
You should find your music in the
Library Home under the USB block.