It's possible to convert a SD file to an HD file as you suggest here but the conversion won't improve the look of your image. It will just divide your SD image up into additional pixels.
In your editor, change your timeline properties to the HD setting you want. This will shrink your SD footage in your preview window. I'm a Sony user so it may differ in FCP but I open my footage in my pan crop window and crop the converted image to 16:9 from the preset. This will blow your image up to the HD size. Problem here is that you'll lose some quality of image by blowing it up even though you'll render it as HD and tape it on to your HD camera. There are software packages out there that claim to make a true conversion from SD to HD but the reviews have been mixed.
Here a link to an article that covers the FCP workflow for what you are suggesting
http://library.creativecow.net/articles/ross_shane/aja_kona3.php