1. I don't believe there is an import utility. That being said, if you're a persohn who enjoys scripting and you could figure out how to dump the contacts from horde for each person, you can import them into zimbra easily through the command line. See
This thread for a bit of info regarding that
2. As long as users have access to the gal via their cos policy they should already have this. The global address list is an address book for all intents and purposes that contain all your system users, or can be tied to an ldap directory. Then, users have their own Contacts folder by default. Is this what you need?