Code:
on *:text:*:#channel:{
  if $server == irc.a.net && # == #a {
    var %i 1
    while $scon(%i).server {
      if $scon(%i).server == irc.b.net { 
        if $1- isin abcde 12345 {
          scon %i
          msg #b $nick $1-
        }
      }
      inc %i
    }
  }
}

New username: hixxy