Turning an obj file of a human into a Poser human figure that bends well is not an easy job. I know people who have spent years developing a figure to the stage where it works as well or better than the figures that ship with Poser. However, if near enough is good enough, it need not take years.

As a first step, each part that needs to rotate, toes, foot, shin, etc, needs to be a separate mesh group in the obj file. For a human figure the groups should follow the standard Poser naming convention, you can find this in the "StdHierarchy.txt", use your OS's search function to find it.

Once you have that sorted, you need to add bones to turn it into a figure, there are several ways you can proceed at that point. You can import the obj into Poser, then use the "Create New Figure" in the Hierarchy Editor, or you can copy the bones from an existing figure in the Setup Room, or there is also a PHI file method. I have never made a human figure, I will leave it to those who have to fill in more details.