As you can see from many existing SCN posts, there is no user exit or any other "kosher" option to add custom fields in FB02/03. Creating another SCN post with the same question will not help with this, unfortunately.
In addition to some BADIs mentioned in the old posts (although all those discussions are abandoned, so I guess those are dead-ends), you can search in the SAP notes for more recent information. Also there are known methods to locate BADI/user exit (can be fund on Google), but it seems you've already tried.
However, it's not really a good practice to use the accounting documents to capture additional data (and it's one of the reasons no user exits are available). You might want to reconsider the design here, perhaps this data should really be captured in some other place.
There is also a standard Text field that can be used for additional information.