Using BON1 must come before using workers/priests for digging: "action BON1. dig 2. transform A1" is right, "dig 2. action BON1. transform A1" is wrong.
This is according to the rules:
If the Spades gained via a Power action or Bonus card do not suffice to transform a Terrain space into your Home terrain, you may exchange Workers* for the missing Spades (at the current Exchange rate of your Exchange track).
(*The Darklings pay 1 Priest for each missing Spade instead.)