Yes, GL_14 only for leading ledger, that is by design . To extract any special ledger, you need to create a Generic DataSource . There is a Tcode for the same . Have a look at this official document.
After creating it, I guess you need to enhance it with MATNR from BSEG.
To enhance with BSEG fields, please have a look at the SAP note I supplied earlier . BSEG being a large table , you might have performance problem in CMOD exit. Using BAdi can avoid performance problem .
Regards
Anindya