It's not possible.
You can use a workaround with the export and create a batch file the launches the program
Example:
$filename(Mp3val.bat,ansi)"c:\Porgam Files\mp3val\mp3val.exe" $loop(%_filename_ext%)"%_path%" $loopend()-lout.log
I was looking how to use the tool manager to use mp3gain from mp3tag and I found this thread...
But I can't find the way to make mp3gain opening with the batch... Doesn't even work from windows...
Here is what I changed but it doesn't work. Can you please help me. Thanks