I have an action which has about 40 steps. It creates about 20 layers. It only runs when the document has a single flattened layer. The document which I wish to run it on already has multiple layers. If I copy the contents of the document via copy merge to a new document, I can run the action. However, I can't figure out a way to get the newly created layers back to the original document.

Am I missing something blindingly obvious?

drag and drop?

Open both documents at once,  click and drag the layer you want over from the source document to the destination document.

That's all that's necessary.

Thanks!

You can also shift-select all the layers you want copied and then right click "duplicate layers" and select the document you want said layers pasted to.