You can Add an Existing Product to an Order, Add a Custom Product to an Order, and Add a Surcharge to an Order with as many products as you wish. The only limitation is that you must add them all at one time.
For example, if someone places an order and then you add a suborder to it with a few more items, and they process the payment for it, you can't add another one later. However, you can create a new order and Merge Orders with Order Manager.
Bold Order Manager is no longer available to install on the Shopify App Store. If you already have Bold Order Manager installed, you will be able to continue using it as it is grandfathered on your store.