Adding Hitch for Bike carrier on rear??

Take it to a custom fabrication shop, if they don't think the frame is strong enough they can use gusset plates or other additions to strengthen it. They can then add a receiver stout enough to handle the dynamic load of that much weight bouncing.

The other thing you could do is add a front hitch to your tow vehicle, that's what I did to carry 4 bikes. It has the added advantage of returning weight to the steer axle.