Asfar as I know, you can't change manually the drainage network density. But you can set up a threshold for the drainage density based on your DEM (you probably already know that). Hope someone else can help you

Thank you Gloria. I know about how to change the threshold.

Maybe you can try this : Under Watershed Delineation -Stream Defination, check 'Predifined streams and watersheds' instead 'DEM based'. But I doubt if your own drainage networks would be more dense than the one auto generated from DEM if the threshold is set properly.