In your example, you can play the willpower card as a peril. When you steel (and as a result play a card as a peril) you can divide any single remaining pool of resolve you have, even if that resolve doesn't match the one required by the card you're playing as a peril.
You can always do this unless you cannot steel your remaining heroes' resolve any further., i.e. the only resolve left on all of your heroes (or the remaining heroes) is 1/1/1/1 or less.
I hope that makes sense!