I am looking at building an external site with a CMS, probably Drupal or ExpressionEngine. The problem is that our company already has a membership database that is designed to work with our existing enterprise software.

Migrating data from the database manually is not an option as modifications and new data must be accessible in real-time. Because the design of the external database will differ from the CMS's own I have decided the best way forward is to use two databases and force the CMS to use the external to read user information (cannot write to) and a local for everything else the CMS needs to do (read + write).

Is this feasible with these Drupal or ExpressionEngine? Ideally I need to be able to use hooks as I do not wan't to modify core CMS files. Sifting through the docs I am not able to find what I would hook into for ether CMS.

(Note: I know it is possible, but I want to know if it's feasible).

Finally if there is a better way of handling this situation please also chime in. Perhaps there is something at the database level to reference a field or table in an external database?

I'm clutching at straws someone can point me in the right direction I'm sure.