How to define grayed out SAP Table fields as characteristic in a Opt Concrn

Hi All,

I need to add some of the SAP standard table fields as the charactersitic in my Operating concern that are grayed out when trying to create a characteristic using the standard tables availble. e.g. I want to add VBKD-FKDAT as a characteristic but it is grayed out and doesnt let me add it. Can any one help me with this. I am new to COPA.

Thanks

MKR