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.

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.

When Transmission announces, 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.

Scraping the tracker simply means asking for information about the swarm itself. For example, the number of peers, seeds etc.