Mac computers come with the Automator application.
Open it.
Create a new “Service” : File > New > Service.
In the library of actions on the left sidebar, you will find one called Run Shell Script : add it to the workflow.
Set the service to receive selected files or folders, the shell as /bin/zsh, and the input to be passed as arguments.
Then enter /Applications/Atom.app/Contents/MacOS/Atom -n $@ in the text area.
Finally, save the workflow as “Open in Atom”.
And you are done !