How can I utilize netlist & device files to place a self-created PCB package symbol

I am a new user of Allegro PCB editor of OrCAD 16.2. I want to modify an existed PCB design (*.brd) without schematic.
My question is how can I utilize netlist & device files to place a self-created PCB package symbol.

I think I need to add my package symbol to Allegro's library. To make Allegro recognize it's name. Then when I modify
netlist file with the name, Allegro will link to the *.dra file which I created.

If my thinking is correct, how can I let Allegro to recognize a new symble.

thanks