SupahNL, I don't suppose the author of this snippet is "alive" these days...probably gone for good. He was last seen active 7 years ago.

Anyway, here is the modification I made according to your request:

alias setallowed {
  var %file = file.txt
  if (!$read(%file,nw,$nick)) {
    write %file $nick
  }
  else echo 4 -at The nickname already exists!
}
on @*:join:#:{
  if ($read(file.txt,nw,$nick)) {
    msg $nick Welcome to # $+ !
    halt
  }
  inc $+(%,wk,$network,#,$site)
  if ($($+(%,wk,$network,#,$site),2) > 3) {
    ban -k # $nick 2
    unset $+(%,wk,$network,#,$site)
  }
  elseif ($($+(%,wk,$network,#,$site),2) == 3) {
    kick # $nick You're not permitted on this channel. $&
      This is your last warning!
  }
  elseif ($($+(%,wk,$network,#,$site),2) == 2) {
    kick # $nick You're not permitted on this channel.
  }
  else {
    kick # $nick You're not permitted on this channel.
  } 
}

It's the same command as the original snippet here, enter /setallowed . Now, make sure you change "file.txt" in the code to the one of your choice. Place all the allowed nicknames in the chosen text file one after another vertically.