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.
ppdxdaniela 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
