The wheel group isnt so that they "cant" su ... its so that they dont have to. The sudoers file specifies that a user in group wheel can do everything that root can do
# Uncomment to allow people in group wheel to run all commands
# %wheel ALL=(ALL) ALL
# Same thing without a password
# %wheel ALL=(ALL) NOPASSWD: ALL
thats from the sudoers file. You can edit that w/ visudo if you want to enable the group wheel to be able to do everything. If you're running slackware, the group wheel is already created. When you create an account through the adduser script, it will ask you for additional groups for the account to be part of ... you could specify wheel there. If you want to make a user part of group wheel ... do a man on usermod.