Postgresql 9.0 adding new database to streaming replication
Hz.Root, your question has many open ends but I will try to answer.


  What is the best way of doing this?


Just create a new database in master server and populate it.


  I have a backup file about 20GB. should i restore to that cluster?


Yes. To be precise, you restore it to the database, not to the cluster. 


  can standby server have all the data? 


Standby server MUST have all the data - that is it's purpose. Every database, and every other object that is in primary PostgreSQL cluster, will be replicated to secondary server.