There is no command line interface for back annotating from .swp file. Form 16.3 release onwards, you can however write tcl scripts to accomplish this task. The .swp file is a text file; for details of the syntax you can refer to Capture user guide. You can read this file through TCL and perform the necessary operations through TCL to update the designs. For details of Capture TCL interfaces, you can refer to the document OrCAD_Capture_TclTk_Extensions.pdf, which is located at tools\capture\tclscripts folder of OrCAD installations.
From memory the command you need is :
back /path/pst back.dat
Where path is a realtive path from your cds.lib directory to your packaged directory (or a full path if you can be bothered).
Take a look at the concept hdl manuals, there is a very useful command line reference manual. You'll find the "back" command in there.