The Feeds SQL module will allow you to use the Feeds module to import data directly from a database. It can be configured to use a custom SQL query that will fetch the data, then you can use whichever other processor on it to create nodes, users, etc.
Right now it will allow you to use any database that has been configured in the settings.php file, but I'm planning on expanding it to user-entered database connection strings.
This module has a basic mechanism to verify that there is a single query and that it is a
SELECT statement. Still, be careful when executing arbitrary SQL queries on a production site.
To test the query, use the SQL Parser pre-fetcher. This will allow you to verify that the query is returning the data you expect. Also, the returned data is used to populate the Field mapping list.
If you have the Token module you will see a list of possible replacement patterns for your query.