http://gbatemp.net/index.php?threads/405921/The emuNand partition MUST be at the beginning of the SD card and must have a special flag set that a PC/Mac cannot see. There is no trivial way to do it without formatting the card. Also, rxTools can inject an emuNand form a backup, it is painfully slow and takes over 1 hour on an O3DS compared to just minutes on a computer.
No formating needed