Just create an item that is a non-inventory item called (for example) "UPGRADE/POWERBOMB"
Add your PowerBomb movement as normal (on its own GameObject).
Add a MustHaveitemCondition to the same gameObject. Set the must have item to"UPGRADE/POWERBOMB".
Et voila: a power bomb that only works when you have the relevant power up.
Items don't have to be items. If an item is non-inventory and you don't add a UI item for it, the player will never see it.
So you can use items for upgrades or even as 'flags' that make up part of a quest or puzzle. For example in a dialog you could grant the player an item "QUEST/SPOKETOROGER", and then you can use that item condition to enable other things in the game to build parts of your quest that are only available after you speak to Roger.