The default morphs won't help you here.
Short of re-rigging the mesh, the easiest way is to use the Poser 7 morphbrush to make your own morphs.
This way you can work directly on the mesh inside Poser while it is posed on the bench.

Pose your figure on the bench.
Make the bench invisible, then flatten the buttocks a little more than necessary with pull/screen and push/surface.
Make the bench visible again, then "unflatten" the buttocks again with "restore" set to a low value until they contact the bench.
Then"spread out" the buttocks with pull/screen at a low value.
Finally use the smooth brush if necessary and save as a new morph.