Generally, anywhere where you see a username you can right-click on the user and add them to your contact list: search results, chat rooms, browsing, etc.

Additionally it would be useful have the option to add a user directly to the contact list after being provisioned with their key via another channel (email, text, IM, etc.)

This technique removes the need to a) know at least 1 file the user is sharing, or b) both be concurrently present in the same chat room in order to add that user to your contact list.

Would this assist anyone seeking to create and index (via browsing) users files en masse in any way ?

Fopnu already supports user links.

To get your own user link, find yourself in a chat room and right-click and Copy Link.  

OR find yourself in a search, right-click and Browse yourself, then the button at the top with your user name in it can be right-clicked, where you can Copy Link.

OR add yourself to your own contact list.  Then you can right-click yourself at any time and select Copy Link.

TO OPEN A USER LINK: right-click one of the main view buttons (eg. Network) and then select Open Links.  A user link can be pasted right in there.  OR if the link is in an email or web-page, you can simply click it and it should open in Fopnu.  For example, fopnu:user:m5ny4nryqqbfaewh4bidqxvtw3kk3rsyi2ag6arae6ohospk4rqq/papa%20smurf%20is%20real