Problem:
You try reusing an SSL from an old server but the application (eg RDS), doesn’t recognize the SSL. When you try to import it, it looks for a .pfx and you can’t seem to generate it.
Solution:
Go to the old server and export the SSL (eg, from the SBS server you go to MMC, add the Certificate plug in and export the remote.domain.com) as a pfx. Make sure that you include the private key.