You can easily add them by following the instructions on their site.
On immutable fedora it can be done via
curl -o - https://repository.mullvad.net/rpm/stable/mullvad.repo | sudo tee /etc/yum.repos.d/mullvad.repo
rpm-ostree uninstall mullvad-vpn --install mullvad-vpn
# after reboot, if not working
sudo systemctl start mullvad-daemon
They don't require an account (username/password) for you to use. You pay them for an account number and use Mullvad VPN by inputting it.
They were about to be raided but they managed to get out of that with their lawyers.
It's also very easy to use on Linux because there's a GUI, which is great. CLI seems overkill for an app that needs to be turned on and off (i.e. NordVPNJ my old VPN).