You are speaking about the slow ballad "Hallehlujah". Well to me slow songs are a little tricky to add too. You don't want to add too much or you will take the song too far out but at the same time you can add runs and passing chords to slow songs easier than the fast ones.
Play the melody by itself with out any chords and make sure you understand it first. Then add trills and embellishments to the single line. Incorporate that melody with the chords in the left hand and I think you will find that you can add to any song including that one.
