Can a Photocopy document turn to be original document, if it is marked with a separate superimposed stamp "original” on that document? Or does it have to be fulfilling the requirement of Sub article 17(b)?

Yes - if clearly maked original - it is an original!

Check the Policy statement:

The determination of an "Original" document in the context of UCP 500

sub-Article 20(b)

http://www.iccwbo.org/id415/index.html  

Still applicable.

Best regards

Kim Christensen