Hi,
In any case, talk to SAP support, explain what you are trying to do, ask for advice and for possible alternatives.
If the "real need" to extend persistent classes is there (the class CL_PT_REQ_ATTABSDATA mentioned above has what looks like generic predefined attributes intended for customers use at least from EA-HRRXX 6.00 SP 0080 onwards) , by the looks of it, it would have to be a modification. Do ask SAP what will happen with such modified objects during upgrade, how are they supported in modification adjustment tools.
cheers
Jānis