How to import rotobrushed videoframes from AE to Animate while preserving the alpha channel?

Hi there,

I have a very short video of a man walking down a street. I used the rotobrush in AE to isolate the man from the street background so now he's now just walking 'in space'.

Now I want to bring this rotobrushed walking man into Animate as a series of still frames (I'm aiming to work at 12fps in Animate) and then draw some scenery behind him, frame by frame (in black, with a plain white background). See attached for a rough sample frame.

What settings should I use to export from AE, and import into Animate? So far, the 'transparent' PNGs I've managed to import into Animate from AE somehow have black backgrounds, and all end up in the same keyframe on my Animate timeline.

Thanks to anyone who can help ease my pain!