See as per the rules and norms by Hons or any full time course , its not the rule to pursue any other course from that university.

Also DU have no such mentions of certificate course can be or cannot be done. Thus its better for you to contact the CU authorities in order to know in details about it.

For the contact details you can copy paste the link below:

http://oldweb.du.ac.in/index.php?page=contact-us

I hope this answer helps. All the very best for your future endeavors!