MP3s don't join natively. (Well, I suppose they do, but it's not tidy.) Any program that claims to do so is almost certainly going to convert them to waveforms, concatenate them, and convert that back into MP3. Which is not what you want. The best way to do it would be to rerip the CD into WAV files that are a more appropriate length for you, then manually convert those WAV files into MP3s. How to do the first part of that depends on your ripper, and may not be possible with the specific program you're using.