attributes as enhancements are not supported for persistent classes

Hello Experts,

To add two custom fileds to meet a customer requirement in Leave Request (infotype 2001), I need to enhance standard class

CL_PT_REQ_ATTABSDATA.

This class is used in carrying data from wd abap screen fields to r/3 tables.

I need to add two Z atrributes as well as need to add their getter/setter methods.

As soon as I go to add attribute, I get the below error:

"attributes as enhancements are not supported for persistent classes".

Is there any way out? Do I need to obtain access key to modify the class? enhancement is not possible?

Thanks

Arindam