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Converter classes are not offered by code completion in <class> element in persistence.xml.
There was another issue related to converters as I remember, it's not supported in internal model yet, only entities, I suppose in this term it's enhancement request.
see also 237603
This old bug may not be relevant anymore. If you can still reproduce it in 8.2 development builds please reopen this issue. Thanks for your cooperation, NetBeans IDE 8.2 Release Boss