The thread has a reply which mentions:
"There are some restrictions on system forms & system items (you can read about it in the SDK help under "Restrictions"). One of these restrictions is that you cannot add new column to the columns collection on system form. However, you can add UserDefinedField using the Application: go to "Tools->Customization Tools-> User Defined Tables - Management"."
I want to know where can I read about the SDK restrictions. I was unable to locate any such topic or document.