<p>Yes. To add trackers to a currently running torrent, go to Inspector -> Tracker, and click the plus button (+). To remove them, click minus button (-). Multiple trackers can also be added to torrent files you create. Each newly-added tracker will be placed in a new tier.
<p>Transmission supports multitracker torrents. All of a given torrent's trackers are divided into 'Tiers'. A tracker from each tier will be contacted simultaneously. Transmission randomly chooses a tracker from each tier until it finds one that works.
<p>When Transmission <i>announces</i>, it is updating its presence to the tracker and asking for more peers. This happens periodically, at the discretion of the tracker, however can be manually invoked via Transfers menu >> Update Tracker.