Hi Gerd,
I had a short look on transaction CJI3. In the selection screen you can choose in menue "Extras" the "Data Source". Without switch PS_CI_SFWS_UI_1 (enabled in switch framework SFW5) you can only choose between "Database" and "Archive" and you have to select single files for accessing the data.
With the switch enabled you could use the archive information system. For that you have to activate some of the information structures for PS_PROJECT in transaction SARJ (like SAP_COBK_PROJ01).
That's all I can tell. I have never archived PS_PROJECT.
Regards,
Benedikt