How to add a Chain, Residues and atoms in pdb file?

Question

I want to add a chain with all the residues and one carbon-alpha atom per residue using the OpenMM. Here the link of instances can be used openMM documentation . Currently I have one chain in my pdb file pdb file. Here is what I've tried:

from simtk.openmm.app import *
from simtk.openmm import *
from simtk.unit import *
import numpy as np
import mdtraj as md

# Here I remove all other atoms except carbon-alpha and stored them in a new file 'CA_only.pdb'
main_pdb = md.load_pdb('1PGB.pdb')
atoms_to_keep = [atom.index for atom in main_pdb.topology.atoms if atom.name == 'CA']
new_pdb = main_pdb.atom_slice(atoms_to_keep)
new_pdb.save('CA_only.pdb')

# Then I try to add a chain and residue, but no results
pdb = PDBFile('CA_only.pdb')
top = topology.Topology
chain=top.addChain(id='A')
residue = top.addResidue(MET, 0)

I might be using wrong names of residues. But I don't know what is wrong. Could someone have any idea how to resolve this? Thanks!!