Sorry Niseold, can you explain further? Do you have a primary and backup D already in place and want to add two more spans, or are you adding a backup D and a third span?
If adding a backup D, see Telebub's instructions above. If you're adding additional spans, you'll need to change the PRI prompt on the D channel in LD 17 ADAN. At the PRI prompt enter the new span's loop number and the Interface ID (IID) being provided by he telco. The primary D needs to be disabled to do this work, so you might want to do this after hours.