ACT5 and ACT6 + pay extra spades
I'll be quoting from the rule book here.
Page 10 has general rules for using Spades. In general, if you're gaining Spades during an Action phase, you can exchange Workers to finish the job. If you gain the Spades outside of the Action phase (such as for the Cult Bonus), you can't.

Game Board Power Action Spaces
For spaces 5 and 6 you can exchange Workers for Spades.

Page 17: If these Spades do not suffice to transform a given Terrain space into your Home terrain, you may exchange Workers* for the missing Spade – at the current Exchange rate on your Exchange track. If you only need one Spade to transform a Terrain space into your Home terrain, you may spend the second Spade on another Terrain space. However, you may not place a Dwelling on this other space.
*Darklings need to exchange 1 Priest for each missing Spade.


Bonus Cards
When using the Special Action on the Bonus card (scroll) to gain a Spade, the same rules apply (page 19).

Cult Bonus Spades

Page 15: You may not acquire additional Spades when getting them as a Cult bonus. You may not save Spades for future turns.