Add DLC to giveaway

? do you mean so the winner gets two keys instead of one you have to send the other one though chat

you can edit the key in the field after they redeem the main game to be the DLC key, otherwise you can add them on steam and send via chat.

You'll want to make a support ticket asking them to add the game to the database. (If I understand you correctly)
Disregard that, see Sundance85's answer below.

I actually remember that case.

None of the Mafia III DLC can be added back because they were removed for a reason.
When 2K upgraded Mafia III to the Definitive Edition all owners and unused keys were upgraded to the Definitive Edition which already contains all DLC.

So basically those DLC can't be added back because anybody who would be able to enter them already owns them.

One of the very few cases when a Steam key turn completely useless. I also have one of those lying around from Humble Choice but there is nobody on this planet who could activate them 😅

As far as I know Mafia III base was changed to definitive edition to everyone, so there is no point to give away the DLCs here. Anyone who redeems the base game will have all DLCs.

For your information, that DLC key you have is useless since the release of Mafia III Definitive Edition, because they basically gave Definitive Edition to everyone who has the base game and replaced it on the store with the DE so no one has the old Mafia III game now.

Oh okay, thank you ^^