For more information on what constitutes a non-managed expiry, check this article.
Adding a non-managed expiry
If you are aware of a non-managed expiry, you can log its details against the customer or lead record in the Expiries tab.
Populated the fields to the best of your knowledge. You can link an opportunity to this expiry from the dropdown.
Expiry work stacks
You can use work stacks of type Expiry to monitor non-managed expiries
Set parameters that best suit your needs. Please note, you will need separate stacks for lead and customer records.