How to use both built-in speakers and external microphone on a single jack?

I would like to use built-in speakers of my laptop *and* an external microphone in the same time (for skype). *But* my sound card has only one connector which does both output and input. So when I plug in my microphone, pulseaudio deactivate automatically the speakers (which are seen as "unavailable" in pavucontrol). How to use both built-in speakers and external microphone on a single jack?