By “built-in connection pool” I take you to mean that using a plain old DriverManager style connection is configured. That is correct, but you can change that to use a DataSource to lookup thru JNDI and thereby get a pooled connection. You do this when you add Hibernate capabilities to your project, or when you create a new config file – the first wizard page of the config file wizard lets you select the method to use.
Configuring the connection pool itself is your job – if you want to use Hibernate to do it, you have to add it to the config file. Generally, under most app servers, you configure the connection pool in the app server configs and just lookup the DataSource via JNDI.
More sophisticated mapping will be coming in a future release (no dates yet).