I found the answer on my own.
1. How can I implement my requirement?
There is a pull request for runtime adding of partitions. The feature should be integrated in the final release of picketlink 2.5.0.
2. In internet I could not find the exact meaning of realm, tier, partition, roles. Can someone explain me what they exactly means, how use them and provide some simple example of their usage?
It was hard, but finally I found what I was searching: http://docs.jboss.org/picketlink/2/latest/reference/html_single