- Make a "temp" folder inside your Music/iTunes/iTunes Music folder
- Whenever you get a new file you want iTunes to manage, e.g. from Handbrake, drag it to that temp folder
- Then drag it from there into iTunes
If you do this a lot, try making an alias to that temp folder on your desktop.
If you *really* do this a lot, leave a comment and I can probably be persuaded to make a folder action script that will automatically send those files to iTunes when dropped there.
(Update, I wasn't the first to discover this.)