Right now, you can add trackers to individual torrents and also automatically add trackers to new torrents. It'd be nice if you could all trackers to all torrents.

In the settings tab under tracker preset, there could be a button "Update existing torrents" or some such.

Above, I meant to say "add trackers to all torrents".

1) In the torrent list select all trackers (Select first torrent, hold Shift and select the last torrent, or just press Ctrl+A)
2) Right click > Trackers > Edit or Search&replace or Pick a preset

Presets can be set in Options > Trackers > Tracker presets.

Although these do not "Add" torrent to all trackers, rather "Edit" them, I don't think an "Add" function would really add anything.
(Except the edge case when you have many torrents with different trackers and you would like to add a single tracker to all of them without modifying the currently used ones. Seems to be an unwise function, especially with private torrents.)

Yes, exactly. I have many active torrents and would like to add a list of trackers to all of them without having to edit the tracker list of each one individually.

I imagine many people could benefit from such a function.

The answer above you is suitable for mass editing trackers.