Hi JPauletti,
At this time we do not have functionality to add a digital signature to Method Screens, including Mobile, even through customization. If this is a must for you and your employees, I can only suggest downloading an app on a tablet/smartphone that would allow you to take in a digital signature, and save it as a picture file (.jpg/.png) (essentially a drawing application would do this). You could then add these files to a standard computer and upload to Method through the Documents Tab. This most likely isn't an ideal solution for you, but would at least allow you to keep a record of it if required down the road, in Method.
-Ben