At Admin Panel -> Settings -> Switches you have a "related topics" option. When this is switched on, a related topics field appears on the submit topic and edit topic pages in the admin options area. Only administrators can set related topics, and must do so by writing out a comma-separated list of the the topic id numbers. You can find topic id numbers in the URL of the edit topic page for a topic, as the id= parameter.

Please note the distinction between this option and the "related URLs" option, which is available to all submitters (not just admins) and allows specifying websites that aren't in the directory.

When you've specified related topics for a topic, the topic details page will display the list of related topics. To customize the display in WSN 9+, go to Admin -> Topics -> Topic Fields and edit the "related topics" field... find the "custom display html" area.