no default option available (yet).
you can specify explicit generator like this:
<!-- table allows you to override/define how reverse engineering
are done for a specific table -->
<table name="ORDERS">
<primary-key>
<generator class="sequence">
<param name="table">seq_table</param>
</generator>
<column name="CUSTID" foreign-table="CUSTOMER" foreign-column="CUSTID" />
</primary-key>
<column name="NAME" property="orderName" type="string" />
</table>
above is from
http://www.hibernate.org/hib_docs/tools ... ml_single/