You need to convert .pfx to .pem first, then convert again without key file, then take out the key file and combine.
- openssl pkcs12 -in file.pfx -out file.nokey.pem -nokeys
- openssl pkcs12 -in file.pfx -out file.withkey.pem
- openssl rsa -in file.withkey.pem -out file.key
- copy /b file.nokey.pem+file.key file.combo.pem