The .itl is simply a listing of what tracks you have selected for that library. It does not include the music itself. I have never imported another library so I don't know if the methods Niel outlines will actually move your music files into the same library folder or simply use them where they are. You can easily check by seeing if they have moved to the folder, or by get info on a track in iTunes and look at the information on where the actual file it uses is located.
Read the links below on what actually makes up a complete library and why combining them involves merging multiple items.
What are the iTunes library files? - http://support.apple.com/kb/HT1660
More on iTunes library files and what they do - http://en.wikipedia.org/wiki/ITunes#Media_management
What are all those iTunes files? - http://www.macworld.com/article/139974/2009/04/itunes_files.html
Where are my iTunes files located? - http://support.apple.com/kb/ht1391