If you want to install additional Python [1] modules inside your Blender [2] environment, you can install PIP using this recommended script [3], then use PIP to install all modules that you want. Note Blender’s Python Virtualenv location is /path_to_blender/blender_version/python/bin/python and you need to use this particular interpreter to launch the script.

[1] https://www.python.org/
[2] https://www.blender.org/
[3] https://pip.pypa.io/en/stable/installing/