You should be able to use any of the available UserDefined fields provided in the Field Mappings to map to an unavailable AX field. To do so, follow the steps listed below:
Step 1: Locate the AX field name for the specified field
Step 2: Modify the ledgerJournalTransPost code in the BrittCustomization class
Step 3: Save changes and map to the specified UserDefined field in the Field Mappings form.