No sure if there is a way to add password using MD5 hash, but TealFrog has tried to create a script for something like this: http://forum.mikrotik.com/t/script-md5-hash-generator/57409/1
I’d recommend just taking the easy road and following Mikrotik’s manual and setting up each user with a different level of access and different passwords: http://wiki.mikrotik.com/wiki/Securing_your_router