How can I enable a user to override a batch for a document?

You can’t override per se, but you can set the Can Forward Documents security setting to allow the user to move an item out of a batch by forwarding it to another stage in the queue:

1. Go to Modules > Security Editor
2. Filter to and select Sales Document Properties
3. Make sure the box is checked, enabling Sales Document Properties
4. Under the Misc sub-settings, select Can Change Workflow Queue and set to True