It is possible to do a DNS check on each registered nick entering a channel but you will notice some users may have two or more DNS addresses depending on there Provider.

Code:
  [color:red]
on *:dns: {
  set %userlevel $readini(ftpxusers.dat, $nick, Level)
  if (%userlevel isnum 1-3) { COMMAND }
}
on *:join:#YOURCHAN: {
  dns $nick
}
 [/color]