Finally the Deployment Guide, chapter 4.2 SLE Maintenance Model, especially section 4.2.3.2 Working with Channels, presented the solution.

During migration from SLED11 SP2 to SP3 wagon/YaST removed the old repositories and added two driver repositories and two local repositories for SP2 (CD) resp. SP3 (DVD). No channel for updates was added with registration. My system was registered and activated. Novell Customer Center shows mirror credentials to a product, for example
https://nu.novell.com/repo/$RCE/SLED11-SP3-Pool/sle-11-i586/

In YaST I configured software repositories according to the list of required channels in section 4.2.3, Channel Model by adding there ncc URLs and channel names. Required was, too, a username and password for connecting to the channels. Username and password are not those of the account, but are shown on top of the user's page with mirror credentials for his product, which I mentioned before.

After these steps YOU worked as expected. I suppose that this solution might apply to other SLE environments.

Other solutions as suggested in the Administration Guide (ch.7) or Deployment Guide (ch.3) or Release Notes or TID 3303599 using YaST, zypper, or suse_register didn't add the repositories.

The system now is up to date.