Do you mean posting to accounting the existing invoices that did not post? No BAPI I'm aware of, but there might be a function module that could be used (at your own risk). Search in SE37 (start with the RV* ones).
Invoices not posted to accounting should be an exception, not a rule, and the users easily can release them in a transaction. So I doubt there would be much of additional functionality to facilitate this process.
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