You can specify nested fetch joins through the query hint,
"eclipselink.join-fetch".
i.e.
JPQL: "Select e from Employee e"
Hint: "eclipselink.join-fetch"="e.manager.phoneNumbers"
Fetch groups do not support cascading yet, although their precursor
partial attribute queries do, you could look into these as well, but they
do not allow caching or updates.
I think an option to force instantiation would be useful without requiring
joining (such as using batch reading), so feel free to add an enhancement
request. You can also add one for nested fetch groups if you wish.