Did you use the Search tool before posting, this is a FAQ, you have to append your fields to RFPOSX, as described in OSS 984305 - Line item: Definition of special fields (T021S), execute RFPOSXEXTEND, then use BTE 00001650 (recod per record) or BADI FI_ITEMS_CH_DATA (whole itab) to fill the new fields.
Regards,
Raymond