How can I assign an existing simple product to a configurable product using the admin interface?

In Magento 2 you need to first use the Create configurations tool to create the simple products to associate, then you need to replace the auto-generated versions with the simple products you actually want.

Yes, there is an option to choose/add/associate existing simple products to existing configurable products In Admin interface.

Follow the below steps:

1. Open the Product -> Catalog from admin interface.
2. Open the configurable product where you actually want to assign/associate simple product.
3. Now, From there configurable product form you will see Configurations option.
4. Click on that option you will see an option "Add Products Manually"
5. Click on that It will open your simple products grid on the popup. Select the simple products which you want to assign.
6. Click on Done on the top right corner, Save your configurable product. You have successfully assign simple product to the configurable product.
7. For more details refer below screenshot :

Go to Product --> Catalog from Admin panel.

Then Click on Configurable Product from Add Product button.

Now scroll down you will see the Configuration section and unfold it.

By default, it will not show Add Product Manually option. So, first click on create configuration and follow all the steps.

It will now show you new product combinations with the attributes options you have selected. Then remove this product.

Now you will see the Add Product Manually option and now you can add all simple products easily. Once it's done click on Save Config.