The Welding Institute, Great Britian, has a booklet Welding Dissimilar Metals, edited by N. Bailey, ISBN 0 85300158 8 which is useful.
Joining steel pipe to austenitic stainless, for any kind of elevated temperature service, does require a nickel alloy filler. This is not only to average out the differing thermal expansion coefficients of the base metals, but also to minimize the decarburized zone at the fusion line of the A 106.
I believe state-of-the-art is to use Inconel 82 bare wire (ERNiCr-3) or Inconel 182 (ENiCrFe-3) covered electrodes.