Currently there are a limit to 5 additional Transaction code/Fields allowed in FE for journal entry. I have found with external system interfaces, additional fields we would like to map to our journal entries for GLA export. I suggestion increasing from 5 to at least 7 or double to 10. They are not required, so those who want/need them have them, but others can keep them simple.
You can associate Custom fields with transactions in addition to Transaction codes. Have you given them a try?