Strictly speaking, you can. To no benefit, though. The individual VRAM/VRM heatsinks are tiny in comparison to even a 120mm rad. Plus the fact that you'd probably need thermal epoxy to attach the heatsinks securely.
Ahh, those heatsinks. Don't bother attaching them to the radiator, they'll do exactly nothing. The frame of the rad really doesn't have any affect on the operation of the fins.
Those heatsinks need a source of heat, so sitting directly above the rear memory chips on the backplate of the 3090, they'll have an affect, especially if you put some thermal pads between the chips and plate. Then the heatsinks will have a dissipative affect.
Otherwise all they'll do is cool off the backplate a little in that area, without really affecting the chips to any large degree. The heat has to physically transfer, air is more of an insulator than a conductor.