I'm still not sure what action you want to have happen when you click on your new context menu choice.
Be careful. Probably worth backing up your registry (set a restore point at least) before doing this. The context menu stuff is all saved in the registry.
Click on the 'TXT Notepadd++Document' line in the window, then click the 'Advanced' button. If you highlight any of the 'Actions' and then click 'Edit', you will see how they are set up. You can add one by clicking 'New' and filling in the blanks. Here is where you'd have to define the action you are looking for, and to help with that you'd have to be clear about what you wanted.
It's possible all you want to do is modify the action taken when you select 'Edit' or 'Open' from your context menu. In that case, simply edit the existing dialogs.
Be careful. Backup first.