Fedora template already has rpmfusion repos you just need to enable them temporarily or permanently via terminal or via the graphical package manager (dnfdragora).
For example, to temporarily enable rpmfusion repos in Fedora Template and install vlc, ffmpeg and mpv:
sudo dnf --enablerepo=rpmfusion-free --enablerepo=rpmfusion-free-updates --enablerepo=rpmfusion-nonfree --enablerepo=rpmfusion-nonfree-updates install vlc ffmpeg mpv -y
Or to install the graphical package manager dnfdragora in Fedora Template:
sudo dnf install dnfdragora -y
Then you can open dnfdragora and enable the rpmfusion repos in the check boxes, refresh, and add your third party software.
Another option is to use snaps in your appvms. Install the qubes-snapd-helper first, and then you can install snap apps in your individual appvms and they will still be there after reboot.
For example:
sudo snap install vlc