I'm using the Synology to download torrents via a RSS feed. Is there any way to create a trigger (such as completed download) oppose to using scheduled task?
That depends on which download program you use. The amc manual does include setup instructions for most of the popular tools.
I'm fairly certain the Synology Download Manager doesn't support calling 3rd party tools on completed downloads at this point. But you could ask Synology to add that a as a feature in the future.