The volume controls on the PSR 500/600 or radioshack clones are not digital controls. They are analog controls so no, you cannot control them directly via computer.
You could run the headphone out into your line in on your computer and control it via the windows sound interface.
The volume controls on the PSR 500/600 or radioshack clones are not digital controls. They are analog controls so no, you cannot control them directly via computer.