Can I give this CD key in Steam Giveaways?

No you cant giveaway free games / dlc from promos.

Keys that have been given away for free aren't usually allowed for giveaways, and there doesn't seem to be an option to create a giveaway for this one.

https://www.steamgifts.com/discussion/kgkUN/free-hooligan-vasja-halloween-dlc

No, this DLC is free

basically if you got it for free in a massive giveaway it can't be used to create a giveaway, people are using money to give stuff here, some use more and give full CV stuff, some use less to give bundle stuff.

A good idea is to read FAQ and Guidelines.