How do I import a .PST into my Comcast email account?
PST files are unique to Outlook. If you have your Comcast mail set up using IMAP, you can use Outlook and configure it for the Comcast account, add the PST file (which will show as it's own set of folders), then copy or move the mail to the Comcast folders. That will then sync with the Comcast servers.
To be safe, use a COPY of the .PST file when you do this, just in case something goes wrong. That way, you can remove the mangled PST from Outlook, then use a another fresh COPY to connect and try again.