@Christos Stefanopoulos,
Do you mean the Ambari users? I'm not aware of any export/import of users. You can do below steps
Get users from cluster1
curl -iv -u admin:admin http://{ambari-host}:{ambari-port}/api/v1/users
Post them to cluster2 using some default password and ask them to change again
curl -iv -u admin:admin -H "X-Requested-By: ambari"-X POST -d '{"Users/user_name":"username","Users/password":"defaultpwd","Users/active":"true","Users/admin":"true/false"}' http://{ambari-host}:{ambari-port}/api/v1/users
You can write a script to read from the 1st API and pass it to 2nd API