adding sounds

Boot Drive/Library/Audio/Sounds is the place to start. Don't know what will/will not work. Give it a try and report back here, please.

 

Short answer - yes, you can.
Firstly you will need to convert your mp3 files to aiff. You can do this with Switch (freeware) http://www.download.com/Switch-Audio-Converter-for-Mac/3000-2199_4-10703967.html or SoundConverter (shareware).

Once you've converted the sound file format, the procedure is as follows:
1. Go -> Computer -> System -> Library -> Sounds
Any added files must be in .aiff format. When adding sound files to the Sounds folder, the process will be blocked by the warning "xxxxx file cannot be moved because ........"
2. Click Authenticate
3. Type in your Password at the next prompt
4. The new sound file will be added to the Sounds folder.

New sounds can then be used via System Prefs -> Sound -> Sound Effects

Have fun!