The designer has a feature called Type Conversion Definitions which are edited and specified through the Type Conversion Definition Editor. Type Conversion Definitions are used to automatically assign type converters to field mappings if the .NET type of the model field doesn't match with the target field's .NET type.
To automatically assign type converters, it's key to specify type conversion definitions using the Type Conversion Definition Editor before mappings are created. To be able to specify type conversion definitions, make sure you have a Relational Model Data storage container for the database in question defined in the project.
Type conversion definitions are ignored unless the Project Setting Auto assign type converter to field mapping is set to true (the default).