parallel aggregation

I have a few questions regarding aggregate functions that would be parallel
safe.
1. do the inputs of combinefunc always go through
serialfunc-deserialfunc or they can come directly from sfunc (worker on the
same machine, perhaps)?
2. can the result of combinefunc ever be fed to sfunc as opposed to other
combiners or finalfunc?

I have in mind a scenario, when a different data structure is used in the
combine stage. For that it would be good if the conversion can happen in
serialfunc-deserialfunc, and the combiner does not even know about the
other structure used for state transition during aggregation. If that is
the case, the only problem remains with the finalfunc. It has to be ready
to receive both types.

Thanks.