Dear Kathrin.
i've made some investigation and i found that Z* fields are added to FTR_00 transacion automatically. I didn't make yet any investigation about std reports like TPM12, TPM13.
If you appended Z field to VTBFHA table they will be automatically added to VTGFHA_WRKLIST (this structure is responsible to ALV grid for FTR_00 report).
So check it.