Here's one way to do it:
1. Open your source layout in Calibre DESIGNrev.
2. In the tree or cells list, right-click on the cell that you want to copy and choose Save as layout.
3. Export the cell to a layout file.
4. Open your target layout.
5. Choose File > Import
6. Specify the path to the exported layout from step 3, choose the import behavior options, and click Import.
7. The "copied" cell will appear in your cells list or tree in the target layout.