There is no option on the DL website to enter a partner number. Until the SkyMiles/WorldPerks merger, months after the actual DL/NW merger, if you prefer NW miles, then be sure not to login to delta.com when buying DL flights. You can add your NW # at check-in, or via online web-chat at delta.com, or if you're really lucky via the DL phones - but it's practically impossible to get DL to get the DL number OUT of the reservation after delta.com automatically enters it if you purchase while logged-in.

Hopefully that gets better after the merger (for other partners) because I believe that Delta has already announced that the nwa.com website infrastructure will be the basis of the new delta.com post-merger. But in the meantime, don't log in!