Sure, iTunes can join tracks, but only if they're consecutive CD tracks, and you haven't imported them. This script allows you to join any two (or more) MP3 tracks (repeat: MP3 tracks only), provided that they are the same format and bitrate.
Also see Join Together, an application that will join any type of files.
(Note: In order to launch this PowerPC-era script on an Intel machine, it must first be opened and then saved using AppleScript Editor, which is located in your /Applications/Utilities/ folder. More information is here.)
Enter text for the selected tracks' Long Description tag
Remove iTunes tracks disassociated from files
Append user-entered text to beginning or ending of selected tag of each selected track
Applet assists with swapping, copying, appending data between track tags
Enter text for the selected tracks' Description tag
Set choice of various number tags of selected tracks incrementally