A user's smime certificate was not backed up, and now resides only in the personal keystore of his computer, and I do not have the password to his account. Is there a way to open his certificate store from MMC while logged into another admin user account?
This is impossible. Under Windows 2000, the Admin account, by default, was a Data Recovery agent.
Under XP, there is no default Data Recovery agent. If you part of an Active Directory Domain, then contact your Domain admin, who will know what account is a data recovery agent.