the Spherize filter may be able to do the trick. Filter > Distort > Spherize, then adjust settings as necessary.
The following page gives a pretty good step-by-step for this:
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http://www.marcofolio.net/photoshop/create_a_fish_eye_lens_e ffect_in_photoshop.html>
I prefer, however, to use the Warp transformation, since it has a Fisheye preset. Select the area of the image that you want to make fisheye (select all if you want) then Edit > Transform > Warp. In the Tool Options bar, select Fisheye as Warp type. You can adjust the amount of warping by dragging the control point up or down.