There are two different issues here.
Issue 1) Does knickerbox allow you to send traffic direct to their site? If they are running their own Adwords camaign, the answer to this will be no, you cannot do it.
I see they are running their own campaign, so I guess they would be pretty annoyed if your ads started pointing directly to their site, as due to Google policy, one of these ads would be removed, possibly theirs.
Instead you would have to set up your own domain, and ensure it has enough diverse content to avoid being considered a thin affiliate, (i.e. just doing exactly what the merchant is doing and offering nothing unique).
This shouldn't be too hard.
Issue 2) Directing traffic to a site of yours, other than that officially registered at Tradedoubler, will cause referrer errors, though the sales will still be recorded normally. You can ask the
TD guys to add any additional domains and the referrer errors will stop.