I had the same problem before, about not being able to to see the content of of fftpack.bin
I read these steps in some other post, but I am not sure if they will work for the PSX version(they should though).
What I did was to extract the whole fftpack.bin file first from the ISO.
I had to use UMDgen to extract it because I didnt know how to use CDmage, but it should be possible with CDmage.
Also, make sure you make a new folder(name doesnt matter), as you will need it later.
After you got the fftpack.bin file, you want to see its content, and to do so you will need to use FFTpatcher for that.
In case you dont know, FFTpatcher is used to patch the ISO itself changing general values like growth multipliers from classes of what items will a shop sell and such.
But aside of that, it got a nice utility(which is what you need right now) to uncompress the fftpack.bin file.
Okay, so when you get your FFTpatcher, open it up and you will see a lot of tabs.
The first row of tabs should be something like: [File] [PSX] [PSP] [About...]
Click on PSP--->Utilities--->extract fftpack.bin
After you click on extract, it will make you locate the fftpack.bin file, and just after that, it will ask you where do you want to save it.
Save it in the newly made folder I told you to make before.
After the extract is finished, you should have around 13 files in your newly made folder.
The files you wanted(UNIT.bin and WLDFACE.bin) are located in Event folder.
Now if you followed the tutorial posted before, you should know how to change the formation sprites/portraits with FFTEVGRP.
Remember that you are using an extracted version of the fftpack.bin file, so after you are done editing to your heart's content save all changes, and is time to compress it in the ISO again.
Open FFTpatcher again, and go PSP--->utilities---->rebuild fftpack.bin
You want to rebuild the whole New Folder you made at the begining, since thats is where you just edited the WLDFACE and UNIT files, so click on that folder and then yes.
After that, it will ask you where do you want to save it and to give it a name.
I dont know if the name matters, but I always name it fftpack just to be safe.
After the rebuild is done, you should have your edited fftpack.bin, so is time to put it into the ISO.
Open your ISO with any program that you use(again, I had to use UMDgen cause I dont know any other) and locate the fftpack.bin file, then replace it with the one you just build.
After that, save it as a new ISO.
This new ISO is now carrying the edited fftpack.bin so when you play with it and go into formation screen, all the changes you made with your sprites and portraits should show up.
Sorry if the steps sound long, but I asure they arent too bad and you will get used to it.
I know I did after having to edit things I didnt liked over and over xD