This is to supplement OP's answer:
According to man pipewire.conf (and a bit of inferencing), you can use drop-in files to the same effect, so this should be the preferred way to do it IMO.
The directories are /etc/pipewire/pipewire-pulse.conf.d/ for global and ${XDG_CONFIG_HOME:-~/.config}/pipewire/pipewire-pulse.conf.d/ for user (which defaults to ~/.config/pipewire/pipewire-pulse.conf.d/ if $XDG_CONFIG_HOME is unset or empty).
So just create this file (mkdir -p if directory does not exist):
${XDG_CONFIG_HOME:-~/.config}/pipewire/pipewire-pulse.conf.d/add-combined-sink.conf
context.exec = [
{ path = "pactl" args = "load-module module-combine-sink" }
]
Note that in my testing, if I put the drop-in file in /etc/pipewire/pipewire-pulse.conf.d/, something weird happens to the combined output (pitch becomes higher and introduces regular stutters).
If I have to guess, it's probably something related to the modules' loading order: most likely there is a way to fix it, but I haven't bothered to investigate. Putting it in ${XDG_CONFIG_HOME:-~/.config}/pipewire/pipewire-pulse.conf.d/ works just fine though, so that's what I did.