If your model is to scale, and you have/make your own model of a scale bar, then you can import the scale bar model to your chunk and merge the models.
In order to place the scale bar where you want it in relation to your model, you would have to import it to a separate chunk first, and then turn on 'show aligned chunks' (though you may have to tweak the scale/position/rotation of the scale bar chunk, using the transform object tools, before it will interpret it as an 'aligned chunk'), and then you can use the transform object tools to put it where you want it. Finally merge the chunks to get a single model including both your scan and the scale bar.
I've not tried it!