When you apply for a certificate of residence, local authority most probably check with the regie if the person involved is mentioned in the rental contract or is known by the regie at the particular address. When not, you don't receive the certificate. Making your own document is useless, as this has legally no value and the regie always needs to know who is living in the appartment. So you have no choice to get in contact with the regie and to explain the situation. Not sure if your roommate needs to be in the rental contract or the regie only needs to be informed. No worries, this is a very common thing and regies don't bite! The regie will definitely not raise the rent because of an additional roommate. Good luck!