I am unable to add custom property definition to a catalog item.
This might be because the datagroup xml files are kept in a write protected location. The following workflow can be adopted to add a custom property definition to a catalog item.
<?xml version="1.0" encoding="Windows-1252"?>
<Version major="1" minor="0"/>
<Units master="m" sub="mm"/>
<InstanceDataDefinition defType="USER" definition="Custom"/>
This will add the new Property Definition (Custom) to the Catalog Item (Wall) as shown in the image below.
To add more definitions to more catalog items, keep on adding from line 6 to line 8 of the code given above with appropriate names for the Catalog item and the definition as applicable. Refresh the DataGroup for the changes to get reflected. Also, the master and sub units can be edited as per requirement.