AddRange to a Collection

Try casting to List in the extension method before running the loop. That way you can take advantage of the performance of List.AddRange.

public static void AddRange<T>(this ICollection<T> destination,
                               IEnumerable<T> source)
{
    List<T> list = destination as List<T>;

    if (list != null)
    {
        list.AddRange(source);
    }
    else
    {
        foreach (T item in source)
        {
            destination.Add(item);
        }
    }
}

No, this seems perfectly reasonable. There is a List<T>.AddRange() method that basically does just this, but requires your collection to be a concrete List<T>.

Since .NET4.5 if you want one-liner you can use System.Collections.Generic ForEach.

source.ForEach(o => destination.Add(o));

or even shorter as

source.ForEach(destination.Add);

Performance-wise it's the same as for each loop (syntactic sugar).

Also don't try assigning it like

var x = source.ForEach(destination.Add) 

cause ForEach is void.

Edit: Copied from comments, Lippert's opinion on ForEach.