How to manually add secret key to KeyStore/KeyChain (Flutter)

I'm currently setting up OAuth1 authentication for my flutter mobile app. This requires adding the consumer key and consumer secret to the app securely.
I'm using flutter_secure_storage at the moment which uses KeyChain(iOS) and KeyStore(Android) internally. The problem is that I can't seem to find a way to manually add the consumer key and secret without putting them in the source code, which obviously isn't an option.
So my question is: Is there a way to manually add said values?