CustomFeed.cs

public class CustomFeed : IFeed
{
    public string EditViewFriendlyTitle { get { return "Friendly title"; } }
    public bool AvailableInEditView { get { return true; } }

    public IEnumerable<IFeedItem> LoadItems()
    {
        var selectItems = new List<IFeedItem>();

        selectItems.Add(new FeedItem { Key = "Foo", Value = "1" });
        selectItems.Add(new FeedItem { Key = "Bar", Value = "2" });
        selectItems.Add(new FeedItem { Key = "Baz", Value = "3" });

        return selectItems;
    }

    // Required for this to work. Seems to be used to identify the feed on the saved form. 
    // If this changes, your dropdown loses it's possible values.
    public string ID
    {
        get { return "9BC5243C-AC09-4E67-AAAB-78D111F57AF8"; }
        set { }
    }

    public string Description
    {
        get { return "My custom data source"; }
        set { }
    }

    public Type ProviderType
    {
        get { return typeof(CustomFeedProvider); }
        set { }
    }

    public string Url { get; set; }
    public string ProviderKey { get; set; }
    public string ProviderSecret { get; set; }
    public string KeyXPath { get; set; }
    public string ValueXPath { get; set; }
    public int CacheTimeout { get; set; }
    public string ExtraConfiguration { get; set; }
    public IFeedProvider AssociatedProvider { get; set; }
}