Review version 0.15.0 which now has automatic DNS handlers for some of the more popular DNS services
The command to use is openssl pkcs12 -export -out name of the .pfx file -inkey name of the private certificate key -in name of certificate -passout somepassword
I can give you a python script that looks for new certificates (outside of certbot) and creates PFX files for them.
I.e. look in
etc/live
Find New Certificates
Create PFXs
Put them somewhere
Andrei