No you can't. Uplay keys can only be activated on Uplay.

But do you really want to gamble on that fact? If it turns out that you can't, then you will get marked as not received and lose a giveaway slot.

As they said, you need to delete giveaway, because it's against the rules. And the other one too, better do it before they end, because if they end that might be a problem for you and winner if he marks as received without receiving it for Steam. And you can't simply delete if they end, you will need to ask support.