I post here the reply to some requests of documentation.
In fact, this extension can be used if you have already a user table. What it requires is to be able to get the username and email from tht table.
e.g. if you have a table that has fields like username, email, password then the query can just be
SELECT username, email FROM yourtable WHERE username='%{user}' AND password='%{pass}'
if your password is MD5 encrypted (usually it is). Then you can use the query
SELECT username, email FROM yourtable WHERE username='%{user}' AND password=MD5('%{pass}')
If the username and password fields have other names in your table then you have to make them as aliases.
e.g. for a drupal site
SELECT name as username, mail as email FROM yourtable WHERE name='%{user}' AND pass=MD5('%{pass}')
Everything else (ACL and Admin status) can be done in Joomla as usual. Once users are authenticated, they become like any joomla users