You may like to check
this thread for a list of free custom-built dictionaries available for all current Kobo models. Dutch appears a few times.
Even though they are not built-in, loading them should be as simple as drag-dropping them to the correct Kobo directory with the correct name.
I'm not a Dutch speaker/reader so can't tell you how good they are. If you want more details perhaps post a specific question in the Kobo sub-forum and perhaps a Dutch speaker can advise you better.