Hi vielflieger. I was thinking the same thing. I posted my membership number into the "Known Traveler Number" field. However, the example given on the Delta profile page is (example: AX123456789US). So I'm not sure if that's right or not.
The other thing is that the e-mail from Delta states:
To opt in and save your information for all future flights, visit your profile at delta.com and:
• Update and save your Secure Flight Passenger Data information
• Or, provide your PASS ID / Membership Number if you are a participant in the U.S. Customs and Border Protection's Global Entry, SENTRI or NEXUS program
The key word is "or". So I'm not sure if any number actually needs to be posted, only updating the Secure Flight Passenger Data.