During summer, it is possible to take a bus to Norddal from both the north and the south, but the bus connection is rare. Please check
http://www.frammr.no/. There is no public transport that can take you to the car park in Herdalsosen, but it is possible to walk from the fjord in Norddal following the asphalt road for 2.5 km and then the gravel road for about 5 km until you reach Herdalsosen (about 500 altitude metres).