Torrent seeding for bitcoin is of course possible, you just have to write the client with built-in bitcoin functionality. However, it borders on piracy (if you're seeding illegal content). Therefore, you have to be extra careful.
I'm pretty sure op was referring to third party services that are already offering such service. Doubt op will make whole client based operation just to earn some dust seeding.
Also, there's nothing illegal in p2p network sharing - it's all about the content of the seeded files.