nProbe can collect proprietary flow fields as explained in https://www.ntop.org/guides/nProbe/case_study/collecting_proprietary_flow_fields.html
This directory contains configuration files for various vendors, so you can use them out of the box with nProbe
Custom Field Value Mappings in ntopng
Custom field values can be mapped to other values. This is the case, for example, of application ids. Typically, application ids are sent as integers as custom field values and it may be useful to have them mapped into their corresponding application name string.
ntopng can perform such conversion provided that an opportunely formatted lua file is created and placed into ntopng folder