You do not need any special tool, other than a text editor, go to the ruleset and modify fusion launcher stats. However as far as i remember the fusion ball launcher has cap of only 1 round, so i think you maybe referring to the small launcher.
EDIT:
I stand corrected, the fusion ball a craft weapon, yes you can just modify its value with no problem.
- type: STR_FUSION_BALL_UC
sprite: 3
sound: 7
damage: 230
range: 65
accuracy: 100
reloadCautious: 32
reloadStandard: 24
reloadAggressive: 16
ammoMax: 2
launcher: STR_FUSION_BALL_LAUNCHER
clip: STR_FUSION_BALL
projectileType: 3
projectileSpeed: 8
just modify the ammoMax line