I guess it would be possible.
You'd had to run 2 Zimbra servers. MX would point to the outward facing Zimbra. Then you'd need a forwarding rule that says, "If this account doesn't exist on this server, sending it to the 2nd server"
Then you'd point the majority of the users to the OSE version and the Outlook users to the Paid for version.
NOTE: This is just off the top of my head, I have never tested.
Ben Franklin quote:
"Those who would give up Essential Liberty to purchase a little Temporary Safety, deserve neither Liberty nor Safety."