Can I attach the main ground from my service panel to the old steel water main in my house?
You would be advised to not depend on the condition of some old discontinued water pipe for grounding. That pipe may have rusted and corroded so much that a lot of it is missing or not even making a low resistance connection into the earth. 

If it was me looking to correct this condition I would contact my local building / electrical safety agency and/or inspectors and determine if you can ground with a copper clad steel grounding rod that you would pound 8 or 10 feet into the ground outside the foundation near the electrical service box. Then the appropriate size heavy gauge copper wire would be run from the service panel to the ground rod.