Some HDRI maps can produce sharp shadows without additional lights.  This does not apply to how IBL is handled in Poser, but it's possible to achieve this with HDRI.  If you scroll down this page to Shadow Sharpness, you'll see the comparison of a tone-mapped HDRI vs. IBL.

http://www.aversis.be/tutorials/vray/vray-high-dynamic-range-hdri.htm

When you don't capture the full dynamic range, you aren't capturing the true environmental conditions, and can't achieve sharp shadows from the sun in the image.  However, properly exposed, high dynamic range images, using only a small area of high exposure, can produce the desired result, without additional light sources.  I don't think Poser can handle these kind of HDR maps.  Would be great if it did.  Correct me if I'm wrong.  This is also the reason most IBL maps cast a blue hue over everything in the scene.  It's due to incorrect lighting balance in the captured image.

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Tools :  3dsmax 2015, Daz Studio 4.6, PoserPro 2012, Blender v2.74

System: Pentium QuadCore i7, under Win 8, GeForce GTX 780 / 2GB GPU.