It took me a while to figure it out myself so I created this Tutorial. You can play the modded ISO in any PSP Environment without any Plugins.
The basic steps are:
1. Extract the GTA3PSP.IMG from the game ISO using UMDGen / PowerISO or any other program
2. Open thy file in GTA Stories...
I couldn´t find anything on Youtube so I decided to make a video. There are other ways using Cheat Engine and Pointer Searchers but this is an easy way to get started.