Sorry that is not supported, it only supports the internal authentication.

To be honest, I'm not a fan of the Sign In module and would never use it myself because I think security features should be tightly controlled and on pages without other content. I implemented it reluctantly because people request it for silly (to me) things like the sign in is "part of the design" but my advice is stick with the main login page and use SSL. If you put the sign in module on pages then you should use SSL on those pages. Big name sites like google usually have a dedicated page for sign in without other things on it and for good reason and I think that is the right way to do things and keep the attack surface as small as possible. 

Best,

Joe