I know you said you didn't want to use an interim step but you can use Access with an automated routine to
1) read the source file,
2) convert the account numbers,
3) export a VI source file,
4) call a VI routine to import the JE.
From a user perspective, it wouldn't take any more steps other than open Access. If you create a form that automatically pops up to confirm the import file and then executes the process, you would be potentially better off.
Just a thought.