While it would be technically possible to manually extract them from the database, forum moderators and administrators do not have that ability. It would require someone like arn with server admin access to pull them out, and it's not something we have any interest in doing.
As you are probably aware, members can report private conversations to the moderators, and so in that case one member of the conversation could make messages visible to forum staff.
With a specific order, law enforcement could undoubtedly also compel us to dig into the database to retrieve a conversation, but that's incredibly unlikely.