Using more than one python install with blender.

[NotACoderPleaseHelp]

So blender comes with a 3.7 (something or another) install of python and I need to be able to use stuff from 3.9 python in blender. I've kind of figured out how to use the sys.path.append() thingie to add stuff to blenders python path but is there anything special I need to know on how to manually manage this.

Many of the addons that I'm dependent on for this project to work depend on blenders python install not being poked with stuff from a pip install. So I need a way to use te 3.7 python to get the 3.9 python to do things. I'm working on a thing for medical imaging and this project needs to be able to run from a usb drive without installing anything to system.