Jonna, the current native distribution of a plant doesn't necessarily mean that the plant is only hardy in those climatic zones. If other saxifrages in Section Irregulares are hardy for you (e.g. Saxifraga stolonifera, S. fortunei, etc.), then it is likely that S. nipponica and cultivars will be hardy for you also.
Malcolm MacGregor (Saxifrages) describes S. nipponica as "one of the more straightforward species in the garden, with the evergreen leaves being able to cope with frosts. It is also able to survive drier and more sunny positions than most of the other species." I hope that's helpful.