Diners Club Mastercard (US) annual fee is $95. Perhaps you are thinking of the Carte Blanche version?

It is still possible to transfer from American Airlines AAdvantage to Diners Club. The only other airline which had offered this option was United, but as you noted the relationship with UA ended when Diners switched to the Mastercard network in North America.

None of this, however, is relevant to flydeu's situation because the card he asked about is one denominated in Euros, not isssued in North America, not a Mastercard and not linked to the USA version of Club Rewards.