I have been playing around with Windows Azure Mobile Services (ZUMO) and found a great article on Using an existing Azure SQL table with Windows Azure Mobile Services by Jeff Sanders. I have been trying to do the same but found some challenges along the way.
If you are using EF Data Migrations on you project you will need to ensure the schema is set to the name of the Mobile Service name instead of dbo (this might be tricky for some); or use the EF DB First Model approach. In addition to that you will need to make sure the column types being used by your app are compatible with the types for ZUMO. Lastly remember to set the id (Not ID as it is case sensitive) column to be an identity column.
With those small changes you will be able to access your previously created tables via ZUMO while minimizing impact to any legacy webapp you may have developed earlier.
Hope this helps
Moving an Access DB to Windows Azure Mobile Services by Scott Klein