There a couple different ways to do this, you can as mentioned by Marc, simply cut the head off of the stalk and then super glue the head to a new rock, cover with bridal veil or some type of soft mesh (not to tightly) and rubberband the mesh to the rock. the old stalk will sprout a new head, and the cutting will eventually attach to the new rock and you can remove the netting.
The other option is to slice the head of the mushroom in half down the center line and attach the new piece as previously mentioned. Make sure you make an even cut with a clean razor blade through the mouth of the mushroom for your best chance of success.