Oinion skin

If you could give us some more information, it would be helpful.

These two boxes are the same animation. The "faded" (gray) one is a clone and controlled by the onscreen control at the bottom. The other one is the "natural" animation (behaviors - random motion, spin, and edge collision). I made the "onion skin" animate slightly ahead of the actual animation.

The clone can be used as an onion skin for the animation since any frame of the animation can be displayed at any location of the playhead — IF — I understand what you mean by an (animation) onion skin.

If you mean something else, please try to be more specific about what you need.

Otherwise, to set this up:

Take the Group with the animation (especially if behaviors are used) and make a Clone. [Set the Opacity to about 30%]

With the Clone, make a Replicator. Set the Shape to Point. In Cell Controls, turn off Play Frames.

You can either add Source Start Frame to a Rig > Slider. Set the 100 value to the number of frames of your project (Source Start Frame should go from 0 to #of project frames). Or, if you need an on screen control, you can draw a Rectangle and add a Poke filter. Add Clamp behaviors to the Poke Center X and Y. If you use a Horizontal control, then set the Clamp to Min = 0, Max = 1. Set the *other* Center control to Min = 0.5; Max = 0.5 (this will keep the OSC nailed at the mid-point of the Rectangle.)

In Cell Controls > Source Start Frame, add a Link Behavior, set the Source Object to the Rectangle and the Source Parameter to Filters > Poke > Center > X (if horizontal). Set the Scale to the number of frames divided by 100 — so if your project is 300 frames long, then set Scale to 3. [If you need an explanation for this, I'll give it a "try"... ]

Maybe this will be clearer:

The white square is the position of the animation at a set frame (playhead position). I'm dragging the onscreen control to view the animation for the frames just before and after the current frame. The white square has a behavior animation so that is a continuous progression, but if I were keyframing up to this point, the onion skin layer would "end" on the last frame.

(The "control" rectangle containing the Poke filter control is located just offscreen at the bottom of the canvas.]

The point I was trying to make (but couldn't really show in the first animated gif) is that you can create a copy (clone) of your "cell animation" and create an onscreen control or parameter slider to view any frame of the animation, before or after the frame you are working on (without moving the playhead). The "onion skin" can be made from almost any level of the document (depending on how you group your animations which covers the scope of one object all the way up to your entire project. You could use it the other way - where you use it to set up a "stop frame" reference for the rest of the project. It really doesn't matter which way it's used.

The Clone > Replicator > Controls (as a self-contained group) can be made into a kind of "package" that can be saved in Favorites for reuse — all you have to do is change the source of the Clone when you reapply it to a project. You shouldn't have to rebuild it from scratch every time you need one.