C# 3.0 Collection Initializers - Incomplete Feature?

I was really confused when I learned that "collection initializer" is so limited. I thought it was so natural to support extension methods for "Add" method, but it doesn't.

In short, collection initializer is the following syntax:

var list = new List<int> { 1, 2, 3 };
var dic = new Dictionary<int,string> { { 1, "a" }, { 2, "b" }, { 3, "c" } };

Each item in the braces is simply transformed to the call of Add method with appropriate signature.

var list = new List<int>();
list.Add(1);
list.Add(2);
list.Add(3);
var dic = new Dictionary<int,string>();
dic.Add(1,"a");
dic.Add(2,"b");
dic.Add(3,"c");

It's a bit more complex in reality, but it doesn't matter here.
So, when I first saw description of the feature I thought: How cool! I can specify extension method "Add" for "StringBuilder" and get the following:

var sb = new StringBuilder { "Text = ", Text, ";", "Value = ", Value };

But, restrictions are too strong - type being constructed should implement IEnumerable and have instance method "Add". IEnumerable is not of a big deal, but inability to use extension methods for Add is deal breaker.

How about this?

XmlDocument doc = new XmlDocument
{
  new Node("person") { Attributes = { { "name", "John" }, { "age", "42" } },
    Nodes =
    {
      new Node("phones") { Nodes =
      {
        new Node("number") { Value = "123 45 678" },
        new Node("number") { Value = "777 77 777" },
        new Node("number") { Value = "987 65 432" }
      }
    }
}

No way. You can't use collection initializer for XmlDocument, because it doesn't have Add methods. It is IEnumerable, though.

May be this?

var set = new PermissionSet( new ZoneIdentityPermission(...), new PrincipalPermission(...) };

No way. PermissionSet is IEnumerable too, but does have AddPermission instead of Add method.

If it only were searching for candidates as follows:

• If type implements "IEnumerable", look for instance Add method with appropriate parameters,
  
• If not found, if type implements "IEnumerable", look for extension method Add with appropriate parameters,
  
• If not found, look for extension method AddToSequence with appropriate parameters.
  

This change, or something similar, would turn the otherwise very limited collection initializers into really powerfull object initialization feature.