Using head light as bar light? Lately I have been using a head lamp, installed on my helmet, while commuting to work.

For improving safety, and considering that I have a bar fixture for a handlebar bag which is unused for my commute, I was thinking of having a 3D printed thingy to attach the head lamp to the steering bar via the fixture.

Is there any shortcoming on this?
It depends on the beam shape. My commuting head torch has a narrow beam. While I mainly use it in addition to a handlebar light I have ridden with just the head torch, and found that I'm constantly scanning the beam. You can't do that on the bars. Other head torches aren't much use for illuminating the road in front, as the light pattern is too wide, but some would be perfect (often the more expensive ones). 

Unless your access to a 3d printer is free, a cheap light might be more cost effective and then you'd still have the head torch. The mount would have to be quite robust and well fitted to avoid vibrations misaligning the light.