I'm using Hibernate with the XML file based configuration.

When building a HQL query do all entities/tables used in the query already have to be included in the XML configuration? In otherwords is it possible to introduce 'join' elements into a HQL query for associations that are not already defined in the XML configuration?

The answer is NO. you can't but with jist one exception

Hibernate does not define (left join on property = property) so you can 't build a join without mapping, but this has an exception (which i will cover below).

Exception for join:
But Hibernate does have a solution for your problem :
You can define your condition in where clause.
something like :
select a.*, p.* from Address a , Parent p
where a.studentNo = p.studentRefNo

So exception is you can have a inner join in this way without mapping.
And right, left and outer jons cant be acheived in this way.