How can I use the generated keypair elsewhere?

Currently, the public and private keys generated are in JSON format.

Is there a way to convert the generated keys to .key or .pem format so that I can conveniently encrypt/decrypt or sign using the key files outside of RN, for eg, on server side or in openssl?

If not, how should I use the JSON format keys outside of RN?

this lib takes raw format. I would recommend using ursa lib to convert to pem file. if you store data in raw format, you can then decode it using same raw format.