Back when I was breeding african cichlids, I had to remove and reintroduce fish into their tanks all the time.
ie, reintroducing a mouth brooding female, who has just released her fry in a nursery tank, back into the orginal tank with an aggressive alpha male.
A trick I used to get a strong fish like yours to stop (or slow down) attacking a new fish is to trim off his tail. It will grow back within a week or 2, but in that time, your new fish will easily be able to out run him and not be injured. It also gives the new fish a tactical advantage in a fight. By the time your damsel grows out his tail, the 2 fish will have accepted each other.
OR
What I do now when I want to add a fish, is to add 2 or 3 at a time. That way, the resident fish is busy chasing multiple targets, thus his aggression is dispersed.
OR
You can rearrange the rock work, destroying the orignial territories and the home team advantage, so everyone thinks they are in a new tank