Accessing old emails from Plusnet in Mail app Windows 8

Hi, I have a new computer with Windows 8 and have successfully set up my Hotmail and also my (main) Plusnet email account in the Mail app.  I can send and receive email from both accounts successfully.  I want to transfer my old emails from Live Mail to my Plusnet email on the new computer.  I installed the Mail Migration app and this successfully brought the emails in but only as a subsection of Hotmail.  This means that whenever I reply to a Plusnet message, it automatically sends it from Hotmail.  I still have nothing in the Plusnet folder.  Plusnet Assist do not support Windows 8 and I'm completely stuck!  Please help!  Theresa