Multimatte individual RGB channels as precise masks in After Effects


Talking about animations, after rendering my multimatte pass in Vray, I want to use the individual channels in AE as masks. I don’t want the ‘color keying’ effect since is not precise at all.
Let me explain precisely: in photoshop I know how to do that. I go in the red channel (or blue, or green) of the multimatte image, press control and left click and the mask I get there is perfect.
Is there a way in after effect to use the single channels of the multimatte element as pixel perfect masks, like in photoshop?