Chin surgery/genioplasty: is it possible to create a square chin?

Yes, it is possible to make the chin wider with either an implant or multipiece genioplasty. In cases where the chin is widened, a small amount of bone is used to fill the central gap.

A widening bony genioplasty is not going to make a square chin like you have shown. That has distinct corners which can only be created by an implant or adding an implant to a sliding genioplasty. Doing a split sliding genioplasty with a midline graft will make the chin wider, just not square shaped.