I will try to explain the way I do it. It's not necesary to edit the act.img if someone asks.
First open the ide.img with the HXD editor and don't forget to backup your files. For example I want juri/luis grapple wrestling moves and norton/hal melee attacks.
For this change jimmy faction from PLAYER1 to JOCK. Then copy from STAT_PLAYER to Player.act
Search for juri or luis it's the same and paste what you copied from STAT_J_GRAPPLE_A to J_Grappler_A.act...
Then do the same thing the oposite way: copy juri/luis stats and fighting style and paste it over jimmy's ones.
Now where says F_Jocks.J_Grappler.. change the J for a G.
In this part /Global/J_Grappler_A....Act do the same and change it for a G.
It should look like this.
Now save and run bully.
note: you will notice that the grapples will not work at the beggining. Just way a few minutes or keep using it and it should work.