It can't be done without re-encoding the MP3 files. This is called 'transcoding' and
will reduce quality, although you may find the additional loss acceptable under some circumstances depending upon how fussy you are.
What bitrate are these MP3 files encoded at originally? If it's CBR320 or similar then you'll probably get away with transcoding them to something lower without them sounding too bad, preferably in VBR (variable bitrate) as this offers the best ratio of quality to filesize possible with MP3.
LameDropXPd can do this for you very easily if you're running Windows. Download and unzip it then right-click on the window and select "Encoding Options". Move the quality slider to, say, 70% (-V3) and hit "Accept". Right-click again and set "Output Directory" to the destination you'd like the new files to appear in.
You should now be able to drag-and-drop your MP3 files into the window and the resultant files will be in LAME VBR at a typical bitrate of around 175Kbps. If you want them even smaller, just lower the quality setting, although I wouldn't recommend going below 50% (-V5, ~130Kbps) if you want to preserve a sensible amount of quality.
Good luck!
Cheers, Slipstreem.