Not sure if this is what you're looking for, but try this link:
ConvertToSecureString Class[^] on MSDN.
It has some examples there.
Edited: This can't be the right answer :)
On second thought look at the comments at the bottom here
Maybe that's where this mysterious method came from ;)
private SecureString ConvertToSecureString(string value)
SecureString ret = new SecureString();
foreach (char c in value)