Because invoices paid by a manual ACH cannot be voided, the invoices will need to be copied to be re-created in Invoice Approval.
You can Copy invoices from many places, like Search or Bank Reconciliation.
If during bank reconciliation you see outstanding ACHs that you want to pay via Check, and you cannot edit these due to Accounting Periods being Closed, you click on the transaction to Copy it.
Once you click on the Vendor, select Update.
This will open the transaction again, and by clicking the triple dot menu on the top right, select Copy.
You'll be asked if you want to copy the attachments with this new transaction.
From here, update the Payment Method to Check, and re-enter the Invoice Number. Since the invoice number has already been used, you will be required to make this unique by adding a -2 or another method in order to save the invoice entry.
Once all fields are updated, select Send to Approval to complete the entry. The invoice will then appear in the Invoice Approval section.
In order to not affect the current month's financial statements, you will also need to Adjust the current outstanding ACH entries to the same chart of account as the above entry.
In the Bank Reconciliation page, you can either Copy the invoice(s) again, but enter a negative sign in front of the Amount, or you can use the Adj. option and adjust the amount to net the two entries to 0.00.