su command missing

Hi,
Is it possible to install the su command in openwrt/busybox ash ?
I need to run scripts in /etc/rc.local with the environment of the root user, eg

su -lc /scripts/myscript.sh

Thank you

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

Su 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:/$

Thank you Sonnyyu

I had installed useradd and added two users before, I installed shadow-su as per your commands but I found out it has a bug when using "-" sign (Unicode), I see in Barrier Breaker it is solved, but i finally solved remaining on sshpass. I also installed openvpn in client mode to go through the firewall without reverse ssh
Your directions are great as usual, I learn a lot.
I'm trying to document about Attitude Adjustment/Arduino Yun package differences though, to have clearer ideas.