Doing as you propose is unethical, potentially illegal and technically pointless.
Redirecting your DDoS traffic to another site makes you as complicit in the attack as the people attacking you. You do not like dealing with the mess that such an attack causes; why do you think that the sysadmin at any other site will like it more. How would you feel if someone decided you were more deserving of high traffic than they were? You'd think they were a cock, wouldn't you? Well that's you.
As I've already mentioned, you'd be just as guilty of directing a DDoS. While you're not generating the traffic, you are directing it. This is exactly the same as most botnets. The people controlling them aren't generating the traffic themselves; they're just directing it to one place. If it's illegal for them to do it, then it's just as illegal for you to do it.
Thirdly, one of the main problems with a DDoS is using all available bandwidth to your server. If you are redirecting traffic on your firewall, the traffic has already travelled over your transit connect, potentially maxing it out. You then want to send it back over the same link, saturating both directions.
The correct response for a DDos is to drop packets on the floor at your router if you're not saturating your transit. If you are, work with your transit provider to block the traffic before it reaches your link.