A repository on GitHub helped me to find a solution to my question.
I’ve used the shutil module and .spec file to add extra data files (in my case, a config-sample.ini file) to dist folder using the PyInstaller --onefile option.
Make a .spec file for PyInstaller
First of all, I’ve created a makespec file with the options I needed:
pyi-makespec --onefile --windowed --name exefilename scriptname.py
This command creates an exefilename.spec file to use with PyInstaller.
Modify exefilename.spec, adding shutil.copyfile
Now I’ve edited the exefilename.spec, adding the following code at the end of the file.
import shutil
shutil.copyfile('config-sample.ini', '{0}/config-sample.ini'.format(DISTPATH))
shutil.copyfile('whateveryouwant.ext', '{0}/whateveryouwant.ext'.format(DISTPATH))
This code copies the data files needed at the end of the compile process.
You could use all the methods available in the shutil package.
Run PyInstaller
The final step is to run the compile process
pyinstaller --clean exefilename.spec
The result is that in the dist folder you should have the compiled .exe file together with the data files copied.
Consideration
In the official documentation of PyInstaller I didn’t find an option to get this result. I think it could be considered as a workaround… that works.