There isn't a way to just add it to his action bar. Arbitrarily adding boost or roll to any ship is something FFG deliberately removed, as part of their meta rebalancing. BTW, Darth Vader is a monster in second edition. And giving him unlimited boosts would kinda break that. ;)
That said, there is one way to gain boost: Afterburners. It's limited to 2 uses and only after fast maneuvers, but it will work even if you're stressed and it's a free action, which is really powerful! You'll just have to be very judicious about when to use it; you don't have enough charges for a protracted battle where you boost every round (that's FFG being cautious about the meta exploding again).
I hope that helps!