Unable to add event listener and action chain to RSSP because the fragment is non-extensible

Summary:

I am looking to add a validation to the Deliver to Location field in the delivery information fragment of RSSP but I am being restricted as the fragment is non-extensible. I am currently using 25A version and I did go through the 25A extensibility documentation but it said nothing about the use of event listeners and action chains, all that is available is adding validation rules to the fields. Does that mean customizations such as addition of event listeners to the page are still not available or am I overlooking into any of the details?

Content (please ensure you mask any confidential information):