To get the true shape, you won't be able to do it in Pro/E or even Pro/Mechanica. The warp tool definitely won't do it, as it is primarily a 'cosmetic' tool used for conceptual design, and is not meant to illustrate the true effects of bending something.
Pro/Mechanica does not accurately predict plastic deformations, either (it assumes linear elastic behaviour). In order to simulate something like this, you need to move to a more powerful package like ANSYS or LS-Dyna.
If you have an actual piece of square tube that's been bent in front of you, the best way to do this may be to get out your calipers and use Pro/E's surfacing tools to make a reasonable approximation of the resultant geometry by constructing the bent section from curves and boundary surfaces.
Good luck!