I am somewhat confused about the intended joint design.
The pipe is threaded, it's soft lead.(Its soft even when hard) It can be treaded into a stailness fitting, adapter or bung- welded, brazed, or otherwise affixed to the stainless. Otherwise a brass, bronze, copper or other adapter could be similarly afixed to the stainless and the lead soldered to it. I'm not aware of a specific flux permitting lead alloys to wet stainless, however if a Sn flash were to be applied in some way (electroless?) it should be feasable, no? I would also look into the properties of various silver-bearing lead solders. I suspect a solder can be selected which would permit direct bond to the stainless without having to resort to a silver-solder interface between the stainless and the solder.
With Pb the process, if hot, will certainly be defined as a solder, no?
regards
d