No,
it is still not possible, because the dictionary-format used by Sony is a closed one, and no additional dictionaries are offerd (not even if you are willing to pay for it)
The dictionary mentioned with PRS+ is a application (by Clemenseken) done for the PRS500/505/300, it can't be used with 600/x50.
However
kartu has loose announced a stardict compatible PRS+ replacement application, but don't expect results fast. See
here
HTH
Mark