Is there an HO-6 Special (broader) form policy that covers all-risk perils (such as sewer back-up)?
Are endorsements available to broaden coverage?
The HO-6 — also known as condo insurance — is designed to be limited in its coverage. I would look into scheduling your building property on an open peril basis which would give you much broader coverage. If not, I would get a standard HO-3 homeowners policy, as it will have broader coverage. Sewer backup coverage specifically is an endorsement you can add to your home insurance, which may be available for condos as well, depending on your specific insurer and policy.
If you have any questions, don't hesitate to ask.