Hi,
the short and probably most useful answer is : 'No, you cannot.' The longer answer is : Pythonista does not allow custom virtual keyboard shortcuts at the moment. It is however possible to register shortcuts for physical keyboards programmatically - with some limitations/problems (for details see [1]). You can also head over to the recent Python 3.6 and A better keyboard threads where you will find partially a discussion on the very thing you want to do. @omz said he might consider it, but as always things are mostly tied to demand, so if you want to see some virtual keyboard customization you might want to express your whishes there (or more officially on github).
[1] https://forum.omz-software.com/topic/3156/editor-configurability/2