You can just load the island (assembly or cabinet) into the layout with the rest of the kitchen.
If you did not save the island as a separate cabinet or assembly, just open that layout, take the island to the cabinet editor and save it. Then open the other layour and add the island.
Always design cabinets and such in the cabinet editor. Save them in a directory of your choice (you can't save to "Standard") and you can place them into any layout.