Unfortunate. It would have been a stand out feature if we could.I tired loading one of the Switch saves on an old Wither 3 save editor to try and change the patch version to 1.31 like the pc. However it wouldn't even load it, does not seem like theres a way.
I have no real understanding of how these things work but the switch save does seem to be under 1.61 where as pc is 1.31.
Saves on newer patches are not backwards compatible with older game versions while older saves can be brought up to the new version.
Image the switch being able to carry PC saves over and it just not a TV to portable console but PC to portable.