Normally, automodel generates an initial model by transferring coordinates from the templates. However, if you have a prepared PDB file containing an initial model, you can have automodel use this instead with the inifile keyword, as in the example below. (This automatically sets automodel.generate_method to generate.read_xyz for you, which is necessary for this to work.)
# Modeling using a provided initial structure file (inifile)
from modeller import *
from modeller.automodel import * # Load the automodel class
log.verbose()
env = environ()
# directories for input atom files
env.io.atom_files_directory = ['.', '../atom_files']
a = automodel(env,
alnfile = 'alignment.ali', # alignment filename
knowns = '5fd1', # codes of the templates
sequence = '1fdx', # code of the target
inifile = 'my-initial.pdb') # use 'my' initial structure
a.starting_model= 1 # index of the first model
a.ending_model = 1 # index of the last model
# (determines how many models to calculate)
a.make() # do homology modeling