OK, here is a picture: 1. Create a u-shaped mushroom in spline modeler 2. Convert to polygons, remove "closing" of the shape (see white arrows) 3. Add hairs: They appear on the trunk (green arrow) and on the inside of the top shape (red arrow).

As suggested by Eric Winemiller from DCG, simply go in the vertex modeler, select Edit->Return shape (I have a French version, I don't know the exact label in English) and there you are: Hairs on the trunk and on the outside of the top shape. (See picture.)

Thanks to you all.