Hi,
Mass activities have their own logic to generate the intervals which will be used by parallel jobs. The logic is contained in a function module that is attached to a so called parallel processing object. These objects and the function module can be defined in view V_TFKDI (using transaction SM30).
I recommend you copy the standard object AR_ACCT_MX into a Z object and corresponding FM ISU_DYN_MASTERVK_INTERVALS_CLS into a Z Function Module. The you add your the logic to generate the intervals in this FM.
EA26 is configured as a mass activity here:
SPRO -> Financial Accounting (new) -> Contract Accounts Receivables and payables -> Technical Settings -> Prepare Mass Activities
Select Mass Activity Type INVO.
Then select Parallel Processing Objects in the left side window named 'Dialog Structure'.
You will see that for EA26 there are 2 parallel objects. Here you will add you Z object.
Then you can run EA26 using your new Z object which will trigger your logic to create the intervals before the actual parallel jobs are being triggered.
Hope this helps!
Daniel Toba