iTunes generates AAC files using the "Good" setting in QuickTime Pro, and doesn't have any options for using the "Better" or "Best" settings.
I started to try using Quicktime Pro directly to rip my music, but it's a pain in the butt. I ended up downloading (and buying for $7.99) Make Mine MPEG4 at http://www.malcolmadams.com/itunes. It was totally worth it, as it provides a nearly seemless integration with iTunes once you get it set up.
Note that before running it the first time, you still need to go into Quicktime Pro and rip one song. Choose the settings you want (I did 160 Kbps and "Best"), and after that, Make Mine MPEG4 will use those settings.
Oh...and to answer your original question...check out the Make Mine MPEG-4 product page for instructions on how to manually import songs using Quicktime Pro.