Does not exist. If you use PDM logins that are tied to your Windows login, then that password will follow your network password expiration. By the way, that functionality only exists in PDM Pro. PDM Standard only allows for PDM level users with no password expiration.
I was afraid of that.
FYI, I found an SPR that describes this issue: SPR 823692 'Add an Option to set password expiration (password change requirement after period of time) for individual users'