For a space deliminated channel list:

alias Chans {
  var %c,%i = 1
  while $chan(%i) {
    %c = %c $v1
    inc %i
  }
  if $isid { return %c }
  else { echo -a %c }
}

For a comma deliminated channel list:

alias Chans {
  var %c,%i = 1
  while $chan(%i) {
    %c = $addtok(%c,$v1,44)
    inc %i
  }
  if $isid { return %c }
  else { echo -a %c }
}