The old way to do this was by listing the directories in NAMEs= entry of the notes.ini file. This has been obsolete since Domino 5, so it is probably not the way it is done on your server. But see here if you want to learn about it:
https://www-10.lotus.com/ldd/dominowiki.nsf/dx/Names
The new way to do it is with Directory Assistance. See here for information:
https://www.ibm.com/support/knowledgecenter/en/SSKTMJ_8.5.3/com.ibm.help.domino.admin85.doc/H_ABOUT_DIRECTORY_ASSISTANCE.html