I wouldn't say it is impossible, but from my experience i've just seen GTAW + SMAW

9CR steels are very prone to martensitization and cold cracking. I am not a welder so I do not know exactly, but i'd say that doing a root pass with GTAW is easier to control than with SMAW (especially in terms of heat input)

I would say he should stay above 0.035 kJ/inch which is the least amount I've found in the procedures I've used in the past