Can I make a giveaway with a won key?

No

Thanks for the deal.

" Please remember, all games you receive need to be activated on your corresponding Steam account to remain in good standing. Users failing to do so will receive a suspension."

NO - this is not allowed - you can get a suspension for doing this - and alot of people will blacklist you for doing this - please use the help tab and take the time to read the FAQ - tab is at the top of the page
hope that helps and clears things up :)

Here're some useful links for any of the more basic questions about the site that you may end up having.

Site FAQ
Site Guidelines
List of suspension types and their durations

Also, if you'd like to correct the category for this thread, or close this thread, you can do so by clicking the green 'Open' in the top right corner.

If you want to be banned, sure.

Why would you join a giveaway, that someone is nice enough to give people a chance for a free game, just to give it away again?

You're forced to use it(activate) on steam.

There are people that do it, but they quickly get perma banned. So if you don't want to end like them don't do it ;)

But good thing you asked beforehand. It could have cost you a lot.