The current version only has its own user system, but the new version will integrate database, and the new version is under test now, it will support mysql, oracle, and ms sql, you just have to set some parameters of server to use your Postnuke user database.
a sample:
<Mysql>
<database-host>localhost</database-host>
<database-name>test</database-name>
<database-user>root</database-user>
<database-password></database-password>
<user-table>user_info</user-table>
<username-field>userid</username-field>
<password-field>passwd</password-field>
<enable-md5>Off</enable-md5>
</Mysql>