Can we add Case Record as child record to Custom Record?

yes, should be very similar to any other parent/child relationship

1. add a new custom field, to the Case-record(where you'll refer the parent custom record); 2. in its definition, this new custom field shall be of type=ListRecord; 3. pick the custom record which you want to be referred; 4. "tick" the "RECORD IS PARENT"-checkbox; 5. Save the new custom field, of the Case-record(you can configure then, if you want the children to be visible or not, in the Case-record, as a sublist...). Now, you have the parent-child configuration.

Thanks. 🙂