You can't. messageAll is a server-side function and is not accessible by the client. You can however do something like:
function serverCmdHaxMessageAll(%client,%message)
{
if(!%client.isAdmin && !%client.isSuperAdmin)
return;
messageAll('',%message);
}
On the server, then do /haxmessageall single_message_lol but im sure there's stuff out there to do that without the crappy _ things. I just can't be bothered to re-invent the wheel, here.