Grant Root Access To Additional User FreeBSD

Just add user to group wheel with this

pw user mod username -G wheel

For example I add user akhfa to wheel group

pw user mod akhfa -G wheel

If you do it well, if you type

groups akhfa

you will get

akhfa       wheel

Source:
http://www.cyberciti.biz/tips/freebsd-becoming-super-user-su-or-enabling-su-access-to-user.html