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)