Can I steal neutral from fixture for 3-way switch device with no neutral?
It appears pretty clear to me that you can't run the neutral separate from the existing cable, it would be prohibited by NEC 300.3(B) that requires all conductors of a circuit to be contained in the same cable raceway, cable or cord.

We don't know the configuration required for the specific smart switch you are using, at least one model I have used only required one "traveler" between switches, so I was able un-reidentify the white for use as a neutral.