Possible to merge layers (video groups) in video timeline?

I made a character walk cycle animation, but the head, body, and legs are on separate layers--or i should say separate video groups. The "head" video group has 31 frames (layers) of the head bobbing up and down, the "legs" video group has 31 frames (layers) of the legs walking, etc.

Like in Adobe Animate, I need the ability to merge all these video groups into one video group, so these components of the characters are no longer separate. 

But, when I select all the video groups in the layers panel, right click and merge, instead of merging only the video groups, it merges all the frames within each video group into one frame. Therefore, there's no animation; just a single, frozen frame. 

Is it possible to get these video groups to merge, like how it's possible in Adobe Animate? If anything, is there a script to achieve this? 

Thank you.

(Also, converting to frame-by-frame animation will not work for me: when I do so, indeed it appears that all layers are combined successfully (flattened), and each frame is preserved so the animation is intact; however, I need to run a script called "frames to sprite sheet", and for some reason, the script doesn't work. If only I could merge the video groups into a single one, the script works).