Moving mail back from Outlook local to Xfinity/Yahoo Webmail

A family member has been running Outlook locally and I believe they are using POP3 with the option to remove email from the servers. I have been telling them to leave the mail on the mail servers so the email is backed up. Is there a way to move year's of mail/contacts back onto the mail servers so they can stop using the Outlook client/local pst or at least have it synced? Since they are using the Outlook client and never log onto the web mail I don't think they've moved to Yahoo yet.