Try something like:
StringBuilder sb = new StringBuilder();
sb.Append("Some text in here with \"this\" in double quotes");
sb.Replace(@'"', @'\"');
UPDATE:
Based on the updated question, OP needs:
String ex="testing "is" always good";
For this, it sounds like, he wants to have a extra back slash here. No idea why and now I am not going to ask, so try:
StringBuilder sb = new StringBuilder();
sb.Append("Some text in here with \"this\" in double quotes");
sb.Replace(@'"', @'\\"');