Unfortunatelly, JPA Join functionality is based on hardcoded mappings into entities. This approach stay agains agile Daobab policy where relations are used on demand. That's why Doabab Join functionality is strongly limited. If you want to use typical joins, you need to step down to JPA level and do it in traditional way, using @JoinColumn annotations. Daobab is fully transparent and any JPA functionality can be applied.
Cross link between tables
However, join tables is possible with sub-queries. This may greatly speed-up query when sub-query regards to cached Dao.