I don't know if this will let you do what you are trying to do, but it might be worth a little investigation.
1. Open Nero Cover Designer
2. Select "File"->"Paper Stocks..."
3. Select "User-defined Stocks" in the left-hand pane, then click the "Add Stock" button in the middle.
4. Select the newly added stock in the left-hand pane, then click the "Add Item" button in the middle.
5. Select the newly added item in the left-hand pane, then adjust the size and placement in the middle pane.
6. Repeat 4-5 as required.
Maybe this will let you do want you want to do.
You also might want to look into some alternative cover printing software as listed here and in the links it contains: