You can make it manually editing Migrations like this :
public override void Up()
{
Sql("CREATE NONCLUSTERED INDEX [IX_Index_name] ON [dbo].[TableName] ([ColumnName1] Asc,[ColumnName2] Desc,[ColumnName3] Desc)");
}
public override void Down()
{
Sql("DROP INDEX [dbo].[TableName].[IX_Index_name]");
}