Kmail: import mails from thunderbird

I have imported a lot of mails from thunderbird to kmail. At the end I had hundreds of folders, each one containing just one message.
My question is one of the following:

  • is there any way of moving the content of many subfolders to the main folder (obviously not dragging and dropping one by one)?
  • is there any way to avoid the importing of messages all of them in different folders?

I add that I have imported from thunderbird for windows to kmail in opensuse 12.1

Thanks in advance