There is a button in Poser (Depending on what version you have I think)

When you click it it will open a small menu and you can choose to make it Black and white.

You can use the threshold to adjust how many details you want to show. It can be a bit of a fiddling with the dials. As you can see in my image the creature is really dark, but the details on the platform are looking fine. This is because the creature as you can see in the first image have a lot of reflection where as the platform doesn't. So you might have to adjust the material for each object in the material editor to make it look the way you want.

This is after I adjusted the material for the creature, in this case I just lowered the metallic settings, but depending on your model, what materials it uses etc, you might have to make a lot more changes.

You can render and save it in black and white if you render it in Preview and then bring it into Photoshop for fine tuning.