Most people use Audacity, its a free app that allows you to cut and snipe songs. Be sure you download the MP3 add-on so you'll be able to use MP3s.
Once cut and made, just use BlackBerry Manager to put it on your phone. Or just plug your BB up to your PC if you have a memory card and copy the file over. It'll show up just like a thumb drive would.
Posted from my CrackBerry at wapforums.crackberry.com