It is not possible to convert from allegro brd file to PAd,
Because you can import to allegro most of all brd files, but you cant convert allegro to others.
Could I load a netlist file to a PCB file (*.pcb, *.brb) that has not been generated/created from half of this schematic ? That means the PCB has almost the components footprints generated from schematic, and I want to load more footprint components on this PCB.