I recently booked two intra-EU flights (operated by Delta partners) on DL ticket stock.
You have to call DL and hope that you get an agent who knows how to do this (if you don't, call back and try again). Basically, you tell them that you want to book an AF or KLM flight on Delta ticket stock. If they don't believe it possible, you might try telling them to:
- "open the screen for other airlines booking" and then that they need to "specifically add the airline code in the search"; and/or
- pursue "settlement through the 'flights' tab, instead of the 'shopping' tab"
I found the MQD return on these tickets surprisingly low. So, for instance, across two tickets, I paid ~800 USD for a total of ~280 MQD. Moreover, a Skymiles award ticket seemed vastly superior in terms of both cost (an award ticket redemption rate landed at greater than 2cpp for business class (useful if you require lounge access for a long layover)) and MQD earn (I would have earned nearly twice as many MQDs).
I also found that pricing ran ~10+% higher than pricing for the same flight on the AF or KLM website.
That said, if you have eCredits, really want a specific (e.g. direct) flight (that DL doesn't offer for award redemption) or really need to avail yourself of DL ticketing policies and procedures (e.g. free changes, etc...) - with the caveat that your medallion baggage, and free preferred seat selection, benefits may not translate directly (or at all) - then booking through Delta may nevertheless make sense for you.
Hope this helps.