Currently on the form you have a small set list of fields you can map to so even if you add a memo field, you don't really have anywhere to put that data on the Enquiry. It would be great to allow the form to map data into custom fields made on the enquiry
This feature is already possible. you can map form fields to enquiry fields. Then via settings you can even map these fields to opportunity fields (or other) to define what happens when you convert an enquiry.
To help make this clear, we've updated the relevant documentation to clarify the use of Mapped (including custom) fields and Unmapped fields. See:
https://docs.prospect365.com/en/articles/6530983-enquiry-web-forms
https://docs.prospect365.com/en/articles/9310406-configuring-custom-enquiry-field-mappings
It would be super helpful to have all the fields found on the New Enquiries available for mapping on the form so you can map data from the custom fields into the full enquiry. ie allow 'summary' as a field to map a custom field to