Found a solution after some fiddling and reading USB Storage article at DDWRT website[1].
The solution that worked for me was to:
1. Enable JFFS2 on router in order to created a writeable system.
2. Enable SSH on router to work from CLI
3. Use `dmesg` to find usb drives detected, note down their locations
4. Create an automated script described in article[1] which mount those drives to folders created under /jffs (from step1)
Hope this helps...
[1] http://www.dd-wrt.com/wiki/index.php/USB_storage