Hi,
SmartStore uses Entity Framework for database communication. To add columns to an existing table, you need to find the domain class for the table you want in SmartStore.Core.Domain.Common. For example, if you wanted to add another address field (e.g. Address Line 3), you would add the following to the Address class:
[DataMember]
public string Address3 { get; set; }
Once you've done this, you need to create an Entity Framework migration, by running the following from the Nuget console in Visual Studio:
add-migration [Name_of_your_migration]
The final step is to tell Entity Framework to update your database with your new migration. To do this you simply run the following from the Nuget console:
update-database
This will create the new column on the Address table, and add a line into the _MigrationHistory table (which is what Entity Framework uses to manage migrations).