The following code will write to <name of network>.txt in your channels\ directory in the main mIRC directory.

Code:
raw 321:*: .fopen -no channels_ $+ $network $+(channels\,$mkfn($network),.txt)
raw 322:*:{
  if ($fopen(channels_ $+ $network)) .fwrite -n channels_ $+ $network $2-
}
raw 323:*:{
  if ($fopen(channels_ $+ $network)) .fclose channels_ $+ $network
}