Got the news from Aegean on October 26th that one can now buy A3 Award and Tier miles and I tried it out right away.
Tier Miles sell for 25eur per 500 with a max of 2000 for Gold and 1000 for Silver. My kid was 300 miles short to Gold re-qualification, so bought 25 eur worth of Tier Miles. Payment done by credit card, Miles credited instantly and Gold status extended instantly.
We can now bank the miles of the planned re-qualification flight to the other program which I am using as a back-up.
This worked surprisingly nice, easy and quick from their website.