I have tried to do it myself and had to admit it's just not for me so if u want to win some money contact me pr.bouncer@gmail.com
That's not how it works.
You need an ISO of the PSP (not PSN) version, Shishi, and either BMP or SPR files of the sprites you want imported. It's just 'click, click, done' from there. Save data doesn't hold any graphics, or it would be huge for absolutely no reason of any sort.
You'll also need to replace the formation sprites, there is a tutorial on doing this for the PSP version