Costa, great question. 

Firstly any diagrams you create are saved in your folders to be used, we even try to copy the notes you write with them. 

So you can simply choose insert sketch and choose browse media.

OR

You can open the existing plan and go to edit mode and click edit diagram and in the pop-up you have the option to copy the diagram to the clipboard. 

Once you have all the drills you require on your clipboard, you can then start your new session. 

Hope this helps!