The easiest solution would be to go ahead and convert all of your footage to 1920×1080, 23.98 ProRes 422 etc. Just know that full hd ProRes is huge and will probably take up almost 4 times as much storage as your original footage.
In Final Cut the only thing you should have to is change the Aspect Ratio (Go to the Motion tab, scroll down and click the “Distort” tab, then change the Aspect Ratio to your desired size (the 4×3 image will be stretched when it first comes into FCP).