jarsigner can use your pfx file as the keystore for signing your jar. Be sure that your pfx file has the private key and the cert chain when you export it. There is no need to convert to other formats. The trick is to obtain the Alias of your pfx file:
keytool -list -storetype pkcs12 -keystore your_pfx_file -v | grep Alias
Once you have your alias, signing is easy
jarsigner.exe -storetype pkcs12 -keystore pfx_file jar_file "your alias"
The above two commands will prompt you for the password you specified at pfx export. If you want to have your password hang out in clear text use the -storepass switch before the -keystore switch
Once signed, admire your work:
jarsigner.exe -verify -verbose -certs yourjarfile