Install software:

opkg update
opkg install shadow-useradd
opkg install shadow-su

Add user:

useradd sonnyyu
passwd sonnyyu
mkdir -p  /home/sonnyyu
nano /etc/passwd
sonnyyu:x:1000:1000:sonnyyu:/home/sonnyyu:/bin/ash

Substitute user:

su sonnyyu
BusyBox v1.19.4 (2014-09-03 19:18:17 CEST) built-in shell (ash)
Enter 'help' for a list of built-in commands.
sonnyyu@Arduino:/$