The FBL*N transactions use logical database, SDF for G/L (FBL3N), KDF for vendor (FBL1N) and DDF for customer (FBL5N) so either set a logical database to the attributes of your program (and search in online help for statements required to use it like GET) or use a FM like LDB_PROCESS to access LDB data. -> Try also to analyze the LBD thru transaction SE36.
Also inform your functional that there are BTE and BAdI to easily add fields to those standard reports -> Look for BAdI FI_ITEMS_CH_DATA or BTE 1650 (many threads already at SCN) to fill new fields appended to RFPOSX – so no need to create new transactions if only actual requirement is to add some texts.
If you actually want to read data yourself from database table (so BSIS/BSAS for G/L, BSIK/BSAK for vendor and BSID/BSAD for customer) you can then enrich the structure extracted using form GOT_BSEGA of report SAPBSEGA, and a FM as ITEM_DERIVE_FIELDS, then display it with FI_ITEMS_DISPLAY (This FM will call the BTE/BAdI implementations).
Regards,
Raymond