You can add custom column in Asset Balance Report tcode S_ALR_87011964. For this case we need to add 2 column, these columns are ANLA-AIBN1 ( Original asset that was transferred ) and BELNR ( Doc No ).
You can open structure FIAA_SALVTAB_RABEST and find Customizing Include CI_REPRABEST and you can insert those custom column in CI_REPRABEST.
You dont need write ABAP Code to get data for the field AIBN1, the value automatically get from ANLA. You can check Asset Balance Report ABAP Code.
You need to create Implicit Enhancement to write your ABAP Code to fill value to field BELNR. First run tcode SE38 then display RABEST_ALV01 ABAP Program, then scroll to line of event END-OF-SELECTION then double click on subroutine TCOLLECT_FUELLEN then create Implicit Enhancement.
For result you can see new custom column DocumentNo .