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.

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.

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.