Merging New Ducks to the Flock

[DizzyDucklings]

Last year I bought two ducklings and raised them separate from the chickens. When I moved them to the fenced in pen with a walk in coop, the ducks preferred sleeping outside at night and would only go in to lay. They formed no bond to the chickens at all. This typically wouldn't be a problem, but when we let them out of their pen, instead of staying with the flock, they wondered off by themselves into the pasture. Sadly, because they were not in the yard, they were killed by an animal.
This year I have two new ducklings. How do I safely add them to the flock early enough so that they bond with the chickens? And what age should they be when I merge them? Is there anyway to train the ducks to go into the coop at night? Thanks!