C# – AddRange to a Collection

A coworker asked me today how to add a range to a collection. He has a class that inherits from Collection<T>. There's a get-only property of that type that already contains some items. He wants to add the items in another collection to the property collection. How can he do so in a C#3-friendly fashion? (Note the constraint about the get-only property, which prevents solutions like doing Union and reassigning.)

Sure, a foreach with Property. Add will work. But a List<T>-style AddRange would be far more elegant.

It's easy enough to write an extension method:

public static class CollectionHelpers
{
    public static void AddRange<T>(this ICollection<T> destination,
                                   IEnumerable<T> source)
    {
        foreach (T item in source)
        {
            destination.Add(item);
        }
    }
}

But I have the feeling I'm reinventing the wheel. I didn't find anything similar in System.Linq or morelinq.

Bad design? Just Call Add? Missing the obvious?