Here are a list of steps to follow to manually add simple products to a configurable product in Magento2.
1. Navigate to the product in question by going to Products > Catalog > Select Product you want to add simples too.
2. Once in the product scroll down to "Configurations" and click the grey box "Create Configurations".
3. Clicking on Create Configurations will open a new menu from here you will create a new simple product with any attributes.
i. Select the attribute
ii. Pick a Value
iii. Leave Images, Price and Quantity just click next
iiii. Click next in the summary
Now that the simple product has been created you can manually add your pre-created simple products.
4. Next to the grey "Edit Configurations" button you can now click "Add Products Manually".
5. This will open a new menu where you can filter the products showing the simples you would like to add to the product.
6. This will bring up a list of products that match the filter criteria allowing you to tick which products you would like to add to the configurable product. Once you have ticked all the products you wish to add click "Done"
7. The last step is to delete your dummy product you created first from the configurations and save the product.