named String.Format, is it possible?

No, but this extension method will do it

static string FormatFromDictionary(this string formatString, Dictionary<string, string> valueDict) 
{
    int i = 0;
    StringBuilder newFormatString = new StringBuilder(formatString);
    Dictionary<string, int> keyToInt = new Dictionary<string,int>();
    foreach (var tuple in valueDict)
    {
        newFormatString = newFormatString.Replace("{" + tuple.Key + "}", "{" + i.ToString() + "}");
        keyToInt.Add(tuple.Key, i);
        i++;                    
    }
    return String.Format(newFormatString.ToString(), valueDict.OrderBy(x => keyToInt[x.Key]).Select(x => x.Value).ToArray());
}