Yes, you can set on JOIN for specific nicks. You just have to do the check inside the on JOIN.

Code:
on *:JOIN:#yourchannel: {
  if ($nick == Nick1) { msg $nick This is your on join message }
  elseif ($nick == Nick2) { msg $nick This is another on join message }
}

..etc.

Or, if you want to have the messages in a text file, it would be better (shorter) to do this:

Code:
on *:JOIN:#yourchannel: {
  if ($read(joinmessages.txt,s,$nick -)) {
    msg $nick $v1
  }
}


Then, in your joinmessages.txt file, you would put:

Riamus - This is my on join message.
Gwydion - This is your on join message.

..etc.

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