Sorry, this is not possible.
However, depending on
why you were hoping to change pen shape, there may be other options. What was it you wanted to accomplish that requires the "square" pen?
For instance, you may want a sharp square edge for a boarder on a title block. For something like that consider using a fill with the center cut out. This will result in a clean square edged corner.
Lastly, welcome to the forum. I suggest that you fill out a signature (Click "Profile" at the top of the page) and add information about which version on ArchiCAD you are using (see mine below for an example), and some basic information about the machine you are using. This information is often essential to answering questions as the answers can often vary depending on such factors.
Erich
AC 19 6006 & AC 20
Mac OS 10.11.5
15" Retina MacBook Pro 2.6
27" iMac Retina 5K