You want to override the tell command, that has also the w and msg aliases by default.
The class that will override these commands is MyTellCommand (extends PluginCommand).
To do this, you've to set the original command in a state that allows it to be overriden. Also, aliases will be registered directly, but since all the work was done for the first registration, it's pretty simple.
//We are in the context of a plugin
$commandMap = $this->getServer()->getCommandMap();
$commandToOverride = $commandMap->getCommand("tell");
$commandToOverride->setLabel("tell_disabled"); //This prepares the command for the next step, setting up the Command->nextLabel
$commandToOverride->unregister($commandMap); //This changes the current label
//Now, we can register our command.
$command = new MyTellCommand($this);
$commandMap->register("myfallbackprefix", $command);
//And the aliases
$commandMap->register("myfallbackprefix", $command, "w");
$commandMap->register("myfallbackprefix", $command, "msg");