Please share the KB article for generating CSR from F5 with existing private key.
This is not possible. During creation of CSR, Openssl generate private key.
You can't generate private key with help of exiting CSR. I can suggest to choose below options.
Option 1: Generate new CSR and share to CERT provider and get cert.
Option 2: Check with Cert provider if they can consider exiting CSR(they have old csr) and share new Public Cert. Then replace exiting cert to new cert in F5.
Note: Take backup before making any changes for Options2