Standalone example to add new extension fits

appending_table_to_hdu.py

from astropy.io import fits
from astropy.table import Table, Column

# Generating dummy file                                                                
import numpy as np

data = np.array(list(zip([1, 2, 3, 4],
                         ['a', 'b', 'c', 'd'],
                         [2.3, 4.5, 6.7, 8.9])),
                dtype=[('a', int), ('b', 'S1'), ('c', float)])

hdul = fits.HDUList([fits.PrimaryHDU(), fits.BinTableHDU(data=data)])
hdul.writeto('tb.fits')

# ######                                                                               

hdu = fits.open('tb.fits')

new_extension = Table.read('tb.fits', hdu=1)
new_extension['new column'] = Column([11, 22, 33, 44])

hdu.append(fits.table_to_hdu(new_extension))

hdu.writeto('tb.fits', overwrite=True)