Hmm, if it isn't too much trouble, could you explain or link me to an explanation of this function, I have never heard of it; it sounds like it could be useful thought, even outside of this current situation.
Well, what text editor are you using, Notepad, Wordpad, Word? In all of those the command you need is Ctrl + H to bring up the Replace function, though I'm sure every text editor must have one.
What you'd be looking to do is add trainable to most every animal, if I am not mistaken. If that's the case then all you need to do is open up each creature file and replacing [PET_EXOTIC] with [PET_EXOTIC][TRAINABLE]. That'd then make every exotic creature trainable for war, and it takes about 20 seconds for each file, piece of cake. If you wanted to do the same for domestic animals then you just have to open creature_domestic and replace [PET] with [PET][TRAINABLE]
Of course that would mean that some animals that could already be trained end up with more than one [TRAINABLE] tag... but that probably completely harmless. Still, if you wanted to make sure it didn't happen then before you do the original replacing I mentioned you could remove ALL the [TRAINABLE] tags with Replace by replacing [TRAINABLE] with a blank.