To add an existing product to an order in Order Manager, follow these steps:
- From "Shopify Admin", select Apps.
- Select Order Manager.
- Select Existing Orders.
- Select the actions icon.
- Select Add Products.
- Select Add existing product.
- Select Add next to the product you would like to add to the order.
- Select Continue with selected products.
- Select Checkout for the customer or Email customer to checkout.
That's it!