First of all cm:content is not property it is content type which all other custom content type extends. It contains set of basic properties like cm:name.
Now if you want to add some extra property you can create new aspect with your custom properties and then add that aspect to content type. It will be better if you explain your req in detail
Thanks for quick reply. Actually When I inherited "cm:content" content type to my custom content. It has got default "cm:content" property/field attached to it. This property we can see as web-preview on document-details page. I am sorry for not mentioning clearly.