You will need to stitch a few images together; when you load images input the information of the view coverage into the appropriate fields in the popup - this is only in case the popup appears. If it does not the information was probably read from EXIF already.
In the simple interface you will find a Projection tab with a dropdown menu of various image projections. These are your desired projections that the end result will stitch to.
Within, you should find a Fisheye projection. Once the stitching is done the end result should cover 180 by 180 degrees, which is more or less fisheye.
Having said all that I am unsure what you were actually asking about. If you just want to change the shape of an image then I can't help you - I have tried that several times myself and don't know how to do it.