You can cross-seed torrents by adding I2P trackers to the tracker list. What helps is when you upload the .torrent file to tracker2.postman.i2p (the only? i2p public torrent tracker) so that others can find the magnet/torrent link and start downloading. That way people can find the InfoHash and also have trackers embedded in the i2p .torrent file to allow you to find seeders.