Yes. You don't need to create a new ad: you can feature an ad you've already posted.
- Go to your profile and open the ad from your list of ads.
- Select the Spotlight or Premium featuring option.
- Confirm payment via your phone's store.
The boost applies immediately to the existing ad, keeping all its photos, description, messages and favourites.
It's handy if an ad hasn't sold after a few days: a spotlight can revive its views.