Display missing fields after consent authorization has started

See i.e. PSU_ID field. It can be Conditional.
If our database presumes bank will choose REDIRECT approach, but then suddenly decides to use EMBEDDED approach (although psu id was not provided as our DB expects redirect), PSU_ID will be missing as an issue on initial validation. But, it needs to be passed after authorization has started (we are no longer in mock execution mode).
Here, PSU_ID is just an example, we need to be able to ask the user for extra fields even after authorization has started