Adding layers to precomposition

PreCompose the Background Layer. The newly created Composition will appear in the Project Panel. You can drag this new Comp and drop it over one or more other compositions to nest it within these compositions AND all this can be done within the Project Panel. The only issue is that the Background Comp will be placed at the top of the Layer Stack. Unfortunately, AE doesn't allow for layer stacking options when nesting or placing new items into compositions.

You may want to send in a Feature Request to allow for Layer Stacking Options when Adding a Layer/Comp into a Composition. CTRL+/ is the current shortcut for this action. Perhaps CTRL+/+UpArrow to place a layer at the top (current constructs) and CTRL+/+DownArrow to place the newly added asset at the bottom of the layer stack may be a good starting point to get things going. If you post such a Feature Request in the User Voice forum, let us know, here, with its URL and hopefully we can get some votes in on it. HTH

Open the pre-comp by double-clicking the layer in the main comp or by double-clicking the comp in the Project Panel. Add a new layer.

If you want to step through a bunch of new background layers you place all of those layers in a new composition, add that comp to the existing Pre-omp, Drag the background comp's timeline below the pre-amp's timeline so both are visible, lock the Pre-comp Comp Panel so it does not change, then click through the visibility in the background comp's timeline.

This is all really basic workflow stuff you should be familiar with.

I'm not clear what you are asking. Pre-comps are just regular compositions and can be manipulated as such. Your question realyl doesn't make a lot of sense and, no offense, if you're just spewing disjointed bullet points nobody can help you nor would you find anything useful on the web. You need to put in some effort to actually explain what you are trying to do.

Mylenium