A partir da documentação do Openbox :
The Key is the name of the key, such as "a", "space", "Escape", "less", or "F1". You can find the name of any key by using the xev command in a terminal, pressing the desired key, and watching the output from xev in the terminal.
Para responder à sua pergunta
<keybind key="A-Prior">
para a ligação Alt + do PgUp e
<keybind key="A-Next">
para a ligação Alt + do PgDn .