Hello Sebastian,
the primary objective of the accounting migration is to ensure and document the completeness and consistency of the migrated data. This is being achieved using auxiliary migration reconciliation accounts (by default 99... G/L accounts, the actual IDs can be choosen).
Both the migration of G/L account balances and the migration of the various subledger objects (Fixed Assets, Open Items,....) post to these reconciliation accounts, with the expected target that at the end these postings even out, leaving the recon accounts with balance 0.
Migrating the subledger accounts without the corresponding G/L account balances would leave you with non-zero balances on these reconciliation accounts, and will jeoparidize the reconciliation of subledger migrations, making it very difficult to ensure (and document, e.g. for later auditions), the completeness and consistency of the migrated accounting data.
We recommend to migrate the G/L account balances together with the subledger account objects, making full use of the embedded reconciliation logic.
This does not necessarily collide with the additional requirement to have, e.g. for month end comparisons, some month end balances in the system for certain months prior to the migration date (which is, if I understand correctly, also the request here) ....