Adding Velocities from Python Script

Hi All,
I am trying to write a python script which runs LAMMPS as a library with python/lammps interface

from lammps import lammps

During the simulation lets say every 100 steps, I would like to add Velocities to some atoms in the system just before the verlet step.
I am trying to simulate electron scattering and momentum transfer to atoms.
I am using

fix pf  all python/invoke 50 post_force post_force_callback

So this fix will call a function in my python script and I can manipulate forces or velocities of the atom.

So my question is should I just update velocities at this step or should I update forces to add the momentum via

Delta P = Delta F * timestep