The information from the file coil_def.dat is integrated in the channel file when loading data from CTF .res4 or Elekta .fif files.
https://github.com/brainstorm-tools/brainstorm3/blob/master/toolbox/io/ctf_add_coil_defs.m
https://github.com/brainstorm-tools/brainstorm3/blob/master/external/mne/matlab/mne_load_coil_def.m
If you are not reading from a .fif or a .res4 file, you would need to create your own function for adding the coil_def.dat information into your channel file. You can find some inspiration in the functions above.