Just give the Wisp the "Harvest (Gold and Lumber)" ability. The Wisp will then be able to cut down trees like the Peasant, the Peon, and the Ghoul, but the Wisp will have no animation for it, it will just sit there and the tree it's harvesting will be randomly shaking. For any animations you might want for the Wisp while it's harvesting, you will need to trigger them.