Using HTMLAgilityPack to convert links into BBCODE?
See the question and my original answer on StackOverflowSomething like this:
HtmlDocument doc = new HtmlDocument();
doc.Load(myTestFile);
foreach (HtmlNode node in doc.DocumentNode.SelectNodes("//a[@href]"))
{
node.ParentNode.ReplaceChild(doc.CreateTextNode("[url=" + node.GetAttributeValue("href", null) +"]" + node.InnerHtml + "[/url]"), node);
}