Saving Signature inside comment form

Hey guys. Having no documentation is hard.
I need some help because I can't get any further.

I want my user to be able to comment to nodes with an additional drawing field.
Could already add the field to the form with hook_form_alter:

		$form['drawingfield']['und']['#type'] = 'signaturefield';
		$form['drawingfield']['und']['#width'] = '790';
		$form['drawingfield']['und']['#height'] = '200';

The field is there and I can draw in it, etc.
The problem is, it does not save. If I submit the comment, it does not save this new field, what am I doing wrong?

Hope you can help :(