Stefan Koell wrote a guide on how to extend CI class in SCSM 2012. SCSM 2012 provides basic Configuration Manager Database that can be easily customized to meet your needs. This guide will show in some easy steps how you can leverage that easy customization. Read the full article here.
A simple and very useful article on how to use Authoring Tool in SCSM to add more properties to forms. Read the article on SCSM team blog here.
If you want to find out more ways on how to extend the CMDB of SCSM check this article: Extending Service Manager 2012 CMDB Automation and with System Center 2012 and PowerShell.