Multiple Database Connections

[eluser]Johnathan1707[/eluser]
Hey, I'm going to use CodeIgniter on a web app I'm building and was wondering what the best way to do this would be.
I have one database used for the app itself, members details etc.
The user will enter login credentials for their own databases then they can run queries on it from my site. I was wondering if there is a built in way in CodeIgniter to do this and if not is there a way I can modify the database functions to pass in a db connection.
If I were doing it in plain PHP I'd probably do..

Code:

mysql_query('SELECT some,stuff FROM table',$user_connection);

I'd like something to work like that in my app.

Thank you