iTunes. Open the Preferences, and select the Importing tab.
Choose "mp3", and "custom" from the two popup menus and enter whatever bitrate you want in the window that appears. Then drag the mp3s you want to convert into iTunes, select them, and under the Advanced menu, select "Convert to mp3"
Tip: It makes it easier to distinguish the new ones from the old ones if you sort your iTunes Library by "date added" (if you don't see that heading, go to View menu > View Options).