To add user codes on the DSC Powerseries PC1555 you will need to follow this procedure * 5 > 4-digit Master Code > 01 > New 4-digit Code >  and if you would like to add more than one, you will need to change the 01 in the sequence to 02, 03, 04, etc. for every other code you wish to add on the PC1616.

The DSC PC1555 is no longer manufactured by DSC so if you are looking to upgrade to a new alarm panel you can use one of the new DSC panels such as the DSC PC1616, PC1832 or PC1864 security systems.

DSC Powerseries PC1555 FAQ's 

DSC Powerseries PC1555 Alarm Monitoring Options