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Well I'm new and not super great at PS, but the first thing that comes to mind is this:

1.  Make a new layer and put it on top of your photo layer.

2.  With the new layer active, choose Edit>Fill> and I believe then choose your color (which you'd make black or dark grey, or use the color picker eyedropper and sample your "real" shadow in the photo).

3.  Use the brush tool and paint away all of that fill layer that you don't want, until the only thing left is the shadow shape that you do want.

4.  Change the blending mode to perhaps Overlay or something like that.  Try more than one.

5.  Add some blur Filter>Blur> to soften it.

Again, I'm sure the vast majority of members here will come up with a better way, it's just what I would try.