I have a wp site in the root directory all set up as a network. I have another existing wp site in a

I have a wp site in the root directory all set up as a network. I have another existing wp site in a subdirectory. Is there a way to add it to the network - by adding changes to the wp config file and htaccess, or do I have to recreate a new one from the network and import all the data? I searched quite a bit and couldn't find an answer - I feel like the answer is probably right in front of me but I don't see it. Thanks.