@David – do you mean just building a new layer with the same name in the other file?
Just copy/paste an object on that particular layer to another file. Then remove the pasted object.
Before that, check "Preserve Layer when Pasting" (*) in the options of the Layers panel.
(*) I'm using a German version of InDesign, so the exact wording may vary…
Many thanks - I mean copying the entire layer from one doc to another.
Has to be done page by page, setting Paste Remembers layers, then copying and pasting (in Place, most likely). It might be scriptable to do an entire file.
Many thanks Peter.