Hey Daniela, something even better is in 2.8 – multi-object edit mode! Instead of using the addon, you can just select all your pieces and enter edit mode on all of them at once to unwrap as an atlas.
@pdxdaniela A worthy side mention is the UVPackMaster addon. I can’t remember if it is free, but it’s not overly expensive if it costs money. I love using it alongside the suggestion from @jlampel