Q:

Best way to add base class values to derived class?

I have a reference file (dll) containing a class, which i use as my base class:
public class Group
{
    public Group();

    public int Id { get; set; }
    public int League_Id { get; set; }
    public string Name { get; set; }
    public string Nationality { get; set; }
}

(Please note that the Group class contains about 30 entities, i have only displayed some) 
Since i use Entity Framework the ID needs to be unique. i get this data from a API call and noticed that the ID is not unique, i have added my own class.
public class modGroup : APICall.Group
{
    public modGroup()
    {
        modID = 0;
    }

    [Key]
    public int modID { get; set; }
}

This setup works, EF is creating the database.
What i'd like to do is get the data from the API (Which is structured as the Group class), create a new modGroup() and set all data from the API call, without referencing each individual object. 
What i would like to do is transfer data without setting each individual entity.
List<APICall.Group> groupData= _ApiRequester.GetGroup();
using (var a = new databaseModel())
{
    foreach (APICall.Group x in groupData)
    {
        var modGroup = new Models.modGroup();
        modGroup.modID = 0;
        // fill modgroup with rest of variables without doing:
        // modGroup.League_iD = a.League_iD;
        // modGroup.Name = a.Name;
        // etc
    }
}

A:

I would use Automapper to map the two classes together in one call. This type of situation is what it was designed for. All you would need to do is create a mapping configuration and then map the two classes. It's a very simple but powerful tool. e.g. Something like this:
var config = new MapperConfiguration(cfg => cfg.CreateMap<APICall.Group, modGroup>()
.ForMember(dest => dest.modID, s=>s.MapFrom(s=>s.Id));
// etc create more mappings here
);

var mapper = config.CreateMapper();
List<modGroup> modGroupList = mapper.Map<List<modGroup>>(groupData);

