Then configure qjackctl to run the following command after startup. Copy it into "Setup..." > "Options" > "Execute script after Startup":
pactl set-default-sink jack_out
And that is.
Pulseaudio will recognize (through D-Bus) that JACK started, and automatically will route audio to it. When JACK is stopped Pulseaudio will revert to normal routing and start sending audio directly to card again.
So (almost) by default Pulseaudio implements the setup detailed above by mmv-ru.
Otherwise you should compile Jack and enable flag to enable jack_portaudio.so compilation.