It would not be safe for the sea urchin. Sea urchins and the vast majority of other echinoderms (sea stars, sea cucumbers, etc.) are adapted to life in saltwater. read more
You should not put any sea urchin in a freshwater tank at all. Water that is as low-salt as is suited for keeping most freshwater fish alive would be too low in salinity even for the Strongylocentrotus droebachiensis. read more