The Cisco Jabber users are synchronized from your Call Manager user directory, so you can simply query the user directory from CUCM using AXL listUser. If not all of your call manager users are Jabber-enabled, you can filter the returned values so that you only use the ones with
as one of the returned tag values.
See the Cisco AXL documentation for more information about AXL.
You could also construct a SQL query to do the filtering for you and use AXL's executeSQLQuery.