Is it possible to override the source selection after the order is created or during the creation of the shipment?
Out of the box, it is possible to enable Automatic Source Selection by navigating to Stores → Configuration → Advanced MSI and customize the source selection algorithm:
Please, note that after the Automatic Source Selection is enabled, the source will be chosen during order creation, and a customer will be charged for the shipping costs immediately on the checkout page. Additionally, please note that it won't be possible to adjust the warehouse after shipment has been placed according to default Magento logic.
However, it is possible to choose the warehouses while creating a shipment, if Automatic Source Selection is set to No. This way, you can select the most suitable source during the shipment creation process manually: