I'm not an S/4 expert but this must not be approached as "how do we take the same stuff we had in ECC and re-create it exactly as it was but in S4". It's a well-known fact that all the accounting tables have been replaced by one jumbo table in S4. Therefore instead of looking for appends to BSIS/BSAS you should first check what exactly those were achieving in the old system, then evaluate what would be the best way to achieve the same or similar result in S4. Perhaps this is not even required anymore or there is a completely different way to get similar result.
To answer this questions you first need to find out what exactly is the business purpose of this.
