I’‘m quite sure that there’‘s no option to add the bookmarked conference rooms to a roster just like a user. I couldn’‘t figure out where Spark does save the bookmarked conference rooms, but this will likely not be the roster. I did never like the feature as it is implemented to switch between contacts and conferences but on the other hand I don’'t want to see “conference.jabber.org” as a roster.
But a Roster named “Chill out rooms” which contains some rooms located on arbitrary servers is something I want to have.
Maybe even a server group could be possible so an admin can add users to it but instead of seeing who’'s online the group members see a conference room and can join it without much effort.