Adding LDAP authentication to an active drupal site

Issue Summary

I apologize if this isn't the right place to post this question.

I have a drupal site with a current userbase that also have accounts on another system that uses LDAP authentication. The drupal site and the system with LDAP are located on completely different domains.

Some of the usernames may be different but I'd like to authenticate my drupal users against the other system's LDAP. Is it possible to integrate LDAP into an existing drupal site after the fact? How would different usernames for the same people be handled - would they have to be changed locally to match up? Can that be done?