Question : convert .p7b certificate to a .pfx

I have an SSL certificate in .p7b format that I need to convert to .pfx. If I try this through the windows certificate managment the option to expert as a .pfx is disabled.

Trying with openssl I have found the following two commands to do the conversion:

openssl pkcs7 -print_certs -in certificate.p7b -out certificate.cer
openssl pkcs12 -export -in certificate.cer -inkey privateKey.key -out certificate.pfx -certfile CACert.cer

but I'm not sure what key to use for teh esecond command, or what certificate CACert.cer refers to.

How can I convert this key to .pfx format?

Answer : convert .p7b certificate to a .pfx

Turns out it's not actually possible to convert a .p7b to a .pfx without a whole pile of extra information (keys and certificates) that get bundled into a .pfx.