As root, edit */live/persistence/TailsData_unlocked/persistence.conf*, the configuration of the Persistent Storage, and add the *apt-sources.list.d* folder as a feature of type `link`:
For example, to add the `non-free` sections of Debian 11 (Bullseye), backports, and security updates, you can create a file named */live/persistence/TailsData_unlocked/apt-sources.list.d/non-free.list* with the following content:
deb tor+http://ftp.us.debian.org/debian/ bullseye non-free deb tor+http://ftp.us.debian.org/debian/ bullseye-backports non-free deb tor+http://security.debian.org/ bullseye/updates non-free