I've only managed to unpack the 'Game.rgss2a' where you'll find the Data file and you can edit the 'rvdata' files such as 'Weapons.rvdata' within that with a programme. Can't remember which programme that was, but an easy search should do it.
Apprently using 'RPG Maker VX' then can repack / archive these files back into an 'rgss2a'. I am assuming you would replace the original 'Game.rgss2a' file.
This isn't exactly a working science at the moment, or a trainer, but it gives you an idea of the process I've looked at.