I would say that it is probably a bit risky, but get your rig weighed so that you know what the tongue weight is now.
If your trailer is near the 15% end of 10-15% tongue weight, you are already near the 400 lb tongue weight DRY; adding cargo to the trailer will increase the TW more. Add the weight of the extension, bike rack, and the bikes onto that and you are probably well over the 400 lbs rating.
There are options for A-Frame mounts that will add some to the TW; about 60% of the rack and bike weight probably, depending on where the mount attaches between the ball and axle, with the other 40% going rearward to the pup axle. Or you can go with cargo rails on the pup roof with standard roof top racks which places most of the weight directly over or behind the pup axle. DON'T consider a bumper mount for your pup unless it is welded directly to the frame itself as it will likely rip the bumper right off the frame from the leverage.
Rack on our 1007:
A-frame rack on our 19H
ETA: Search for bike rack on this sub-forum. Someone also posted a photo of a rack that attaches the front forks to a rack on the A-Frame, with the rear tires resting on the trailer roof. Another good option for a pup.