Perhaps if you had a plastic welder you can put some tabs inside the bumper to mount the lamp assembly to the non-foglight bumper??
Or you could contact some upfitters for a factory bumper WITH the foglight assembly built in. I just installed mine yesterday. Some Sprinter enthusiasts go with the big metal bumpers and the discarded stock bumpers go in the back of the shop somewhere. My local upfitter even went as far as to obtain the factory rocker switch with raw pins, standard pigtail connectors, and a relay harness for me (all that costed more than what he charged me for the bumper). I went and bought some power cord to make the power connections to the lights. AND, because your "aftermarket" foglights are not hooked into the canbus, you don't have to add resistor harnesses (which get very hot) to fool the canbus when running aftermarket LED bulbs. My Hikari 9600lumen cool white LED's just came today from Amazon. When it stops raining, Im going underneath and replacing the bulbs!