Can you choose to engage a face down enemy in the shadows?
Yes you can.
(Quote from learn to play book p.6)
Facedown enemies are treated in the same fashion as
faceup enemies, but the information on their cards
is hidden. A facedown enemy is flipped faceup in the
• When an effect causes it to suffer wounds
• After it is engaged
• Before it is activated (see “Enemy Activation”
to the right)
Faceup enemies, whether or not they were facedown
before, cannot be flipped facedown.