In order to use Post-Adoption Report fields on Checklists, use an Auto-Action that inserts the PAR Checklist Section when the "Is Current PAR" field is changed to "Yes".
You cannot manually add PAR Checklist Items to an Entity's Checklist, because PAR records are 1:Many per an Adoption record, which means that SAM won't know which PAR record to use and it will just select the latest one.
The "Is Current PAR" field is a special field that is auto-set once the "Completed and Sent Date" is entered. SAM will automatically change the "Is Current PAR" to "No" for the last one and change it to "Yes" for the next PAR based on the PAR Due Dates.
Checklist Section Setup
In order for this to work, the PAR Checklist Section must have the "Post-Adoption Reports" selected as the "Section Table", which is done in the "Edit Checklist Section" screen. This is part of the trick to telling SAM that each PAR Checklist Section is for a different PAR record.
How to Setup the Auto-Action
This Auto-Action should be available in your SAM, but if for some reason it is not, or its been modified, here is how it should be setup: