MicrosoftExtensionsConfigurationKeysToJson.cs

// paste this into a LINQPad C# Program

void Main()
{
	var str = @"Key1=Value1
Key2:Key3=Value2";
	
	var strings = str.Split(new []{"\r\n"}, StringSplitOptions.None)
	.Select(s => s.Split(new[]{'='}, 2));
	
	var result = Merge(strings);
	
	JsonConvert.SerializeObject(result, new JsonSerializerSettings{ Formatting = Newtonsoft.Json.Formatting.Indented }).Dump();
}

Dictionary<string, object> Merge(IEnumerable<string[]> strings)
{
    var dict = new Dictionary<string, object>();
    foreach (var pair in strings)
    {
        Insert(dict, pair[0], pair[1]);
    }
    return dict;
}

void Insert(Dictionary<string, object> dict, string key, string value)
{
  var parts = key.Split(new[]{':'}, 2);
  if (parts.Count() == 1)
  {
      dict.Add(key, value);
	  return;
  }
  
  
  
  if (!dict.ContainsKey(parts[0]))
  {
    dict.Add(parts[0], new Dictionary<string, object>());
  }
  
  var subdict = (Dictionary<string, object>)dict[parts[0]];
  Insert(subdict, parts[1], value);
}