Just use NetworkCredential. It has the conversion logic built-in.
SecureString ss = new NetworkCredential("", "fizzbuzz").SecurePassword;
As others have noted, all of these techniques strip the security benefits of SecureString, but in certain situations (such as unit tests) this may be acceptable.
Update:
As noted in the comments, NetworkCredential can also be used to convert a SecureString back to a string.
string s = new NetworkCredential("", ss).Password;