Generation of a certificate request based on an existing RSA key and certificate
It is not recommended (good practise) to reuse an RSA key pair.
If for technical reasons you need to do so (e.g. Key Pinning), then you generate a new Certificate Signing Request CSR (new_request.pem) based on the existing Certificate (cert.crt) and the associated private key file (private.pem), as follows:
openssl x509 -x509toreq -in cert.crt -signkey private.pem -out new_request.pem
Output the generated CSR in plain text:
openssl req -noout -text -in request.pem