Navigate to the Tasks option on the Runbook you wish to copy a stage to. For example you wish to copy a stage from Runbook A to Runbook B then go to List Tasks screen in Runbook B.

From the Tasks panel click on the Edit button.

Select the Copy Stage option from the available menu.

1. The Source Runbook will be Runbook A (as you are coping the stage from here), select the Source Context i.e. Implementation or Backout depending on the context for the stage you wish to copy is located, then select the Source Stage (this is the stage which will be copied into Runbook B).
2. Select the target position of the stage going into Runbook B i.e. Pre, Imp or Post and then select the Destination Stage.  This is where your newly copied stage will be positioned.  You also have the option to place the copied stage before or after the destination stage.
3. Create a new name for your stage and use the check boxes to retain various elements.  A stage description can also be added at this point.