How to create custom figures?
Step 1: create the mesh (OBJ-File). No ordinary human being can do this within Poser itself, so you need a modeller.
Step 2: import the OBJ into Poser, goto the setup room and set up a skeleton to it.
Step 3: Group the polygons to the belonging bones.
Step 3a: adjust the joints.
Step 4: create a texture.

And this is the reason why most figures cost money because: if you do this all by yourself it takes months. (Except if you make something lik Dr. Geep's Boxman. But this is a very good practise for beginners.)