<p>Yes. To add trackers to a currently running transfer, 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. Currently trackers will be added to a tier before Tier 1, meaning attempts to use your custom-added trackers will be made before built-in trackers.
<p>Transmission supports multitracker torrents. These are torrents which have multiple trackers as a fallback in case one doesn't respond. All of a given torrent's trackers are divided into 'Tiers'. Transmission randomly chooses a tracker from Tier 1 until it finds one that works. If it doesn't it moves on to Tier 2, and so on. This system prevents any one tracker being overloaded.
<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.
