Hi alberto,
Got it, sorry. i checked on my machine also. not showing at right side.
if not showing at right side then next possible way is enhancement and , RSAP0001,EXIT_SAPLRSAP_001, INCLUDE ZXRSAU01.
You need to add FKSTK field to extract str thru append str. once you appended you can unhide added field at RSA6.
Maintain logic at CMOD and Take abap expert help to get proper selections/conditions to fill the VBUK-FKSTK.
you can test at Tx - RSA3 , added field was filling or not. once its filled you can replicating into bw side and remaining steps are know to you.
Thanks