You could try an additive such as 'Cataclean' but I have no verification that it works. Your DPF has probably reached a level of soot that will not passively or actively regenerate. If you get a successful forced regeneration at a dealer, then start running on superdiesel and change the oil and filter more often (more often than every 10,000 miles or every year, whichever comes first). If it's done more than 80,000 miles then the DPF could be clogged with ash (not soot) and needs to be sent away to be chemically cleaned out.