Is there a way to store my own fields in the audit file?

Sure there is!

Suppose you want to add the full path and disk file name of each file. Add a "File folder" field to the audit file, name it as you want and make it as long as needed.

In the global embed "FullRecord: Prime fields for audit file" simply add

AUD:FileFolder = TheFile{prop:name}

Being AUD:FileFolder the new field you created.

TheFile{prop:name} will give you the full path and disk file name of the audited file, if you want strictly the path you'll need to take off the name.