append new scalar field to existing vtk file

I want to get started with python vtk, by reading an vtk file (slice) and then add a new scalar field to the existing velocity magnitude field, and then write out the new modified vtk file (ideally with an image file too). Im not sure whether to use the standard python vtk, or try pyvista instead - it seems more compact. So basically I need be able to get the magU or “mag(U)” scalar and modify it in a simple way such as: newU = magU +10, and save that in the vtk file.

Any help getting started much appreciated, Thanks.