There are some Flatpak apps that have optional dependencies that are not bundled with the app. E.g the plotting package Veusz is available as a Flatpak and it can optionally use the Python package h5py to open HDF5 files. But h5py is not bundled with the Flatpak version. I saw a flatpak command flatpak enter that the documentation says allows us to
Enter the flatpak sandbox.
So I started the Veusz flatpak app and used flatpak ps to get its PID. Then I tried sudo flatpak enter PID pip3 install h5py but it installed h5py in my host operating system!
TLDR; Is it possible to install custom packages (not just Python packages) in a Flatpak application's sandbox? If yes, how shall I do it?