Unfortunately, not through iTunes. I have requested them to allow users to alter the standard 128mbps conversion but no response.
I believe other programmes will allow you to rip two copies at a time i.e. one in lossless and one at 320 mp3.
My solution is to log onto my PC as another user and in that iTunes I add files from my lossless library and then get this 2nd version fo iTunes to convert to 320mp3. Messy but it works - although it does mean I have 2 copies of each song.