An SMTP connector, in this case, won’t help you since all those connectors do is route SMTP mail. If your ISP is using Exchange, ask them to use EXMERGE to pull out all of the emails for your people, and then use EXMERGE to put those PST files into the correct user’s Exchange mailbox. If your ISP is using some other mail server, then I’m going to assume you are connecting them via POP3 or IMAP. You can easily take the PST used for these types of accounts and use EXMERGE to put the data in those PST files into their Exchange mailbox for them. However, you will want to make sure your ISP is forwarding new mail to your new Exchange server, and have your users synch up their email before you EXMERGE their PST into Exchange to ensure all of their email is download and present in the PST.