Thanks for sharing Peter!

By combining your value adding with this answer: https://our.umbraco.com/forum/extending-umbraco-and-using-the-api/97597-creating-custom-data-type#comment-308044

I was able to create a composer to migrate a collection of select lists from a bespoke CMS. Hopefully this helps someone else.

public class CreateDataTypeComponent : IComponent
{
    private readonly ILogger _logger;
    protected IDataTypeService dataTypeService = Current.Services.DataTypeService;
    public static DbMigrate db = new DbMigrate();

    public CreateDataTypeComponent(ILogger logger)
    {
        _logger = logger;
    }

    public void Initialize()
    {
        var select_list = db.policy_select_list_types;
        foreach (var list in select_list)
        {
            string dataTypeName = "Dropdown - " + list.name;
            const string editorAlias = "Umbraco.DropDown.Flexible";

            var exists = dataTypeService.GetDataType(dataTypeName) != null;
            if (!exists)
            {
                var created = Current.PropertyEditors.TryGet(editorAlias, out IDataEditor editor);
                if (created)
                {
                    DataType newDataType = new DataType(editor)
                    {
                        Name = dataTypeName
                    };

                    var values = db.policy_select_list_values.Where(x => x.select_list_id == list.id);
                    var newValueList = new List<ValueListConfiguration.ValueListItem>();
                    var idCounter = 0;
                    foreach (var val in values)
                    {
                        if (val.display_text != "")
                        {
                            newValueList.Add(new ValueListConfiguration.ValueListItem
                            {
                                Id = idCounter,
                                Value = val.display_text
                            });

                            idCounter++;
                        }
                    }
                    ((ValueListConfiguration)newDataType.Configuration).Items.AddRange(newValueList);

                    dataTypeService.Save(newDataType);
                    int newDataTypeId = newDataType.Id;
                    _logger.Info<CreateDataTypeComponent>(newDataType.Name + " : " + newDataType.Id);

                }
            }
        }
    }
    public void Terminate()
    {
    }
}

Works like a charm, I wouldn't know how to include ValueListConfiguration property. Thanks buddy