I would be careful of trying to "design functionality" that just doesn't work in the product. You are working with a COTS product here not a development environment - you are limited to what configuration you can do with the product so you must understand what that is limited to before you start to offer your users some imaginative functionality.
Guesses; (since I don't know much about your "lessons learned" object)
- You will be able to create a subobject of your lessons-learned and create instances of that subobject from within the lesson-learned record (but the UI for that will not be as you have imagines, it will be a separate subpage within the lessons-leaned accessible from the 'Properties' drop down.
- You could create new "Action Items" based on a process initiated from the lessons-learned instance ; again the UI related to this will be very different to what you have imagined.
- You could put a link (not a button) on the lessons-leaned page which takes you to the ActionItems for the project - and from there the user could create a new one.