There is a way to do this in combustion without a plugin.
Select your layer and dupluicate it. Select the new layer and ensure it is above the old layer in the workspace panel.
Apply a discreet keyer to the top layer and select LUMA as your keyer. Now when you switch to the MATTE in the kayer you can see the areas being keyed. By adjusting the tolerance and softness, the keyer can isolate the highlights in the image.
Once you are happy with the isolated highlights switch back to RESULT in the keyer and apply a Blur filter to the top layer. Now viewing the composition in the viewport and increasing the blur you will notice that your highlights are being softened…
Regards
Grant
Discreet UK