You cant reload the mail data files back to Yahoo! mail, but you still can save and backup your data files from outlook. Go to Start>Search>for files and folder, after the search open, click All Files and folder, click more advance option> tick the "Search Hidden Files and Folder, type the search key: .pst
The pst word is with dot, which is the format for data of outlook. Then you will find where is it or where you save it at. After you found it, just copy and backup the "Personal Folder.pst"(Usual will set as this name by outlook) to your notebook or other pc for viewing your email. When you copy it to other pc, after setup the outlook, go to File>open>Open Data File> click at your Personal Folder.pst file then your outlook will load it back. I did complaint bout this to Yahoo!mail before, but seems like they are not support to reload the data back to Yahoo!mail data server, so.. this is the only way which you can save it out for viewing. Other wise is ... :D resend all of your mail ^^