OS X server has very specific uses for Open Directory.
I suggest that you create an Open Directory Master. Along with the it's included accounts.
There's no harm in it, and it will still allow you to connect to other directories.
Then connect to an external server like so:
Open up the Directory Utility App. It's in /System/Library/CoreServices/
Double click the LDAPv3 text.
Click the new button.
If desired, click the manual button and set things up till you're blue in the face.
Go back to the server.app and you should be able to import those users now.