You have two ways:
1. OpenKM 2.0. Enterprise edition will come with database user autentication ( you could use it, but it\'s pay
supported version).
2- If you want to connect to existing database take a Look at this documentation entry point
OpenKM Configuration.
You must change OpenKM login to autenticate to your database ( really simply ).
For full integration you need to extend PrincipalAdapter class ( really simply too ).
If you decide to do the second option, see
Developer guide ( on les 10 minuts you\'ve got running).
Hope It could help you.