While Atelier csp support continues in development, I've been able to get existing csp files over to Atelier as described below.  It takes advantage of the relationship between the .csp file and the generated .cls file described here:

    http://docs.intersystems.com/latest/csp/docbook/DocBook.UI.Page.cls?KEY=...

1) In Studio, bring up the csp file, View Other Code, copy, and paste into a new Class file for the report in Atelier

2) In Atelier, edit the CSPFILE parameter to point to the proper location in the instance being used by Atelier.  In my case, I was going to a Mac, so I changed:

    Parameter CSPFILE = "C:\InterSystems\<windowInstalldir>\CSP\<namespace>\<reportname>.csp"

to:

    Parameter CSPFILE = "/Applications/<macInstalldir>/CSP\<namespace>\<reportname>.csp";

3) Confirm that the CSPURL line reflects the correct path to csp files in the Atelier instance.  In my case, I was using a different namespace, so I edited:

   Parameter CSPURL = "/csp/<namespace>/<reportname>.csp";

to reflect the correct namespace.

4) Synchronize and compile the report class in Atelier.

5) Place a copy of the Studio csp file in the Atelier location pointed to by the CSPFILE parameter

Hope this helps.