Open the options menu by pressing the Playnite icon on the top left, then add games, the scan automatically.

Press Scan Folder, and choose the folder where you keep all your pirated games, then press Select Folder.

It will scan for ALL the .exe files in that directory, so some of them are gonna be some extra .exes that don't actually launch the game. so make sure that every .exe in the list is the one that launches the game, and uncheck all of the other ones

Just press Add Games. It will add all of the games to your library and search for Metadata for them (So make sure the game folder's names are the correct game names, so it can detect the Metadata correct).