CER and P12 are both types of digital security certificates created with the OpenSSL program. P12 is a type of encryption within the more well-known PFX family (it shares the extension). To convert a CER certificate to a P12, simply run one command in OpenSSL. The security data will be the same, retaining your access key embedded within the file itself.
Copy and paste the following into the command window: openssl pkcs12 -export -out OUTPUTNAME.pfx -inkey KEY.key -in FILENAME.cer -certfile CACert.crt
Replace "OUTPUTNAME" with the desired name of your P12 PFX file.
Replace KEY with the access key of the CER (which will be transferred to the P12).
Replace "FILENAME" with the name of the existing CER file.
Hit Enter. This will perform the conversion, saving the P12 in the same folder as the CER.