In order to do this, you must have completed 2 previous tasks:

Defeat Overlimit Chopper in Story Mode
Create the Rumble Ball

After achieving those requirements, you will now be eligible to transform into Overlimit Chopper. To do so, you first must be in a boss fight. While in the fight you have to use TWO Rumble Balls (wait for one to run out, heal SP gauge, take another one). While you are using the second Rumble Ball, the enemy has to kill Chopper. Instead of dying, like he normally would, a cinema scene occurs. The scene shows Chopper taking a third Rumble Ball and he transforms into his Overlimit form. He gains a new HP and SP bar. He isn't able to use items.