Hi
If you are writing your own authentication filter you can use the authenticationComponent to set the user id (see HTTPRequestAuthenticationFilter). If you are using the public APis there is no way to do what you want, nor should there.
I am still not clear what you are doing. Have you already validated the user's encrypted password? This must be done at some point.
If this does not address your problem then please describe what you are trying to do. What are the bits in the authentication stack - what has been done - and at what stage Alfresco authentication fits in. If not, there is not much else I can suggest.
Please note that I respond to community postings in the forum in order, as I have time. Sending private messages and emails will make no difference to response time and will generally be ignored. If you require more urgent help then please contact support.
Andy