Hello!
I guess the answer is similar to the one I provided in the following thread:
community.dynamics.com/.../1062749
With the current Workflow, the posting is not done automatically. It focus only on Approval of the Journal lines.
You should create your own code/Workflow Response that allows the automation of the posting of journal lines once those have been approved, and select it just after the approval has been done in the workflow responses.
You can find information here:
docs.microsoft.com/.../Walkthrough--Implementing-New-Workflow-Events-and-Responses
I guess this can be achieved, but you need a little customization on it.
I hope it helps :).