Skipper is currently not able to handle traits. Traits are Php constructs, not ORM constructs. Because of that, there is currently no way how to handle it in Skipper.
In case you want to handle @ORM fields in Skipper, it needs to be placed in the entity. You can freely place it in the trait, but it will not be visible in Skipper.
We are still considering traits, but as you can see based on this thread, there is the only small amount of users who use it. But such change would require huge changes in Skipper.
In case you're using traits only as a timesaver for Timestampable, I believe there is no issue when you will put these columns to trait. There will be no dependencies to these fields from an entity (associations, indexes,..), so it's not an issue.
On the other hand, instead of using traits, you can use new Skipper feature "Field templates", where you can define columns createdat and updatedat with one click from the entity editor:
These templates are fully customizable so you can define your own or update the existing ones.