Hmm, interesting conundrum there...

Now I'm a little surprised at Poser, considering in Carrara, if you place a light you'll get a shadow unless the light is too far away from the character, or out of range, thus also the character would be much darker also... Unless the ground /floor is too dark (which i've had a few times too). But then you've got highlights in the floor texture, which should pickup the shadow...

Wish I could be more of help...

:edit: I guess Poser doesn't quite want to give up the multi-lighting.

I wonder if you could try adding a negative light and place it so it does cast a shadow, or adding an extra light in the same position as the main one, just for shadow effect.  I long gave up Poser back on version 4. Dunno how much or little help this would be...