I'm assuming you have something like a SS o-let or a boss or a thermowell on a grade 91 header? These things are common in the power industry.
Off hand the only thing I can think of is the precipitation of carbides in the SS. I don't know that this is, in and of itself necessarily a problem. Whatever sensitization that would happen to the SS will still leave it with greater corrosion resistance than the Grade 91.
Stop and think about it. You would have to remove > 50% of the Cr from solution to reduce it to the level of the Grade 91.
But were it me, I'd butter with a nickel alloy. Cook it. Then as weld the SS. Thats the way I've always done it.

If you are talking about a 20" butt weld in 0.898" wall pipe, then you should butter the P91 pipe with Inconel 82 or 182, PWHT, then weld the 304L to the Inconel butter as JS55 mentions.  You could go with 347 instead of 304L to avoid the sensitization concern.  You realize the low carbon grade of 304 is not going to be good for creep resistance at the elevated operating temps that P91 is used for.