Just as in the headline ... can I buy the add-ons here, if I have the base game on steam?

To my knowledge you can't download a DLC without owning the base game here, just like steam won't let you download a DLC if you don't have the base game. So unfortunately no.

What a pity ... but thanks for your answer :-)

It’s a question for steam. Though why anyone would want to rent a game for full purchase price is beyond me. Steam has about as much respect for gamers as I have for s**t on my shoe. Did you actually read the T+C before you clicked ‘I agree’ ?