Marc,
I'm not using 5.6 yet, but in 5.5 when you create a user, you can specify how you want e-mail delivered to that user. Choose to forward e-mail to address below, and in the box underneath it, put in the address of the person. Now, just like in exchange, this user will show up in your address book. The only difference is that the user you put in your system will show up in your address book with an address @yourdomain.com. But since you have set it to forward to this users real mail address, you should be OK.
JB