How this mod works: 
Essentially Each of the characters below have had a skill replaced by a brand new "lock in" skill I've created 
Goku: Body blow
Vegeta: Massive Strike
Piccolo: Rapid-fire ki wave
Gohan: Rapid-fire ki wave (goes for all Gohans)
Trunks: Shining Sword Attack
Gotenks: DIE DIE Missile Barrage
Vegito: Split Finger Shot

What the "Lock in" skill does: 
For example, say you want to use Super Saiyan 3 in free roam. What you would do is go into any battle, and then You would go ssj3 and after you've done that, Use the Special "lock in" skill and it should look like you are about to go ssj2, but you don't, you stay in ssj3. What the game did was Turn whatever form you had on into your base form, it locks whatever aura (except surge auras) and head you had while using the skill to your base. So, once you finish the battle or leave you should still be in that ssj3 form with the aura and all

(Limitless only) Form Stacking:
This is slightly complicated, for those who use TheReckon5's Limitless Expansion pack (https://www.nexusmods.com/dragonballzkakarot/mods/270)
you have the option to Stack your Vanilla forms on top of Reckons Custom forms. 

How to stack forms: 
It's best if I just give an example. So, say you want to stack Ssj on on top of UI sign, you would go ahead and enter a battle, go ssj, use the "lock in" skill, which is body blow for Goku, finish the battle or leave (and you should still be ssj). Then go into Goku's know-hows and select auto ui sign (it only works with auto know-hows (the form things)) and when you go into battle next, he will start to go ui sign but will stay ssj and 
after you surge you should get the ui sign surge animation and surge combo but still remain ssj with the ssj aura.
sadly, you can't seem to stack vanilla forms together, only vanilla and limitless forms or limitless on limitless forms. If that makes any sense,
and yeah, I think that's it. have fun ;)