If you mean a normal garbage dumping zone (i.e. for things designated for dumping), it should be possible to do this by creating one in the normal way - I guess the most sensible way would be to create a channel down from above the magma. You may need to disable any other dumping zones you have to stop your dwarves deciding they'd rather go to the surface.
However according to this wiki entry (spoilers beware), note that "any items dropped into the magma sea (specifically, onto "magma flow" tiles above semi-molten rock) will vanish without a trace".
If you mean for general refuse, I don't think there's a way to dump this automatically. You could keep a refuse stockpile near your magma dump and manually designate things for dumping now and then, but that may be a hassle. Alternatively you could just create a larger indoor refuse pile with appropriate precautions for sealing the resulting miasma inside the stockpile, let the majority of refuse rot away and use any bits that don't in crafting (bones, shells etc).