ah ok. I'm guessing that this MAY be possible, but not without some serious hacking to openGoo. The system is designed to retrieve any emails from the mail server and then store them in your opengoo DB. In order to bypass this and get opengoo to use your mailserver instead of it's DB...
I've never used IMAP, but supposedly it's a more 'direct access' kind of method.
I wouldn't mind using opengoo as the front end for my mail server either.. but I personally don't have the skills to hack and reprogram opengoo until it is. I think this would be a very custom solution.
What about forgoing Horde or SquirrelMail all together and requiring your users to only use remote email systems like Thunderbird, openGoo, gmail or (if you must) Outlook? Then you would have to set up the email for your users (or show them how to do it) in whatever system they are using, but no longer provide direct mailserver access.
This is actually what I do for my company.