Converting a .p12 SSL Code Signing Certification to the .pfx format is amazingly simply. I wasted a couple of hours looking on Google without luck. So I hope this post saves someone else the time I wasted.
1.) A .p12 and .pfx are the exact same binary format, although the extension differs.
2.) Based on #1, all you have to do is change the file extension.
On Windows, open a command prompt and cd to the directly that contains the .p12 file. Then run the following command:
rename CompanyX.p12 CompanyX.pfx