You can import any URL <x id="START_LINK" ctype="x-a" equiv-text="<a href='https://rg3.github.io/youtube-dl/supportedsites.html' target='_blank' rel='noopener noreferrer'>"/>supported by youtube-dl<x id="CLOSE_LINK" ctype="x-a" equiv-text="</a>"/> or URL that points to a media file. You should make sure you have diffusion rights over the content it points to, otherwise it could cause legal trouble to yourself and your instance.
An HTTP request has to be sent on each tracker for each video to spy. If we want to spy all PeerTube's videos, we have to send as many requests as there are videos (so potentially a lot)