Additionally, does anyone know how to include "Voiding Journals" in the approval workflow? It doesn't seem to be a subtype that we can select under the currently placed workflows.

voiding jE are system generated JE so not available for approval process by design

Use a condition based on user creation {createdby.id} = -4

@Marc Wouldn't that condition mean that all other subtypes are subject to the approval workflow (such as biils, bill payments, etc.)?

Exclude in the condition all the record that you don’t want approval wf