can I take biology addition subject after 12th exam in hbse board
You can't add the subject to the score you have given the 12th. If you want to do so then you have to register yourself again with the subject combinations of your choice from the starting. As a result, I would recommend you to contact your school or the Boards regional office in order to know about the regulations required to do so as there are no information available on the HBSE official website.
Goodnight :-)