What type of content do the CDs have? If they are audiobooks, you might want to look at Chapter and Verse, a free utility that allows you to join multiple mp3 or m4a files (which could be imported from multiple CDs) and merge the results as a standard iTunes audiobook (.m4b).
If you want to take music from several CDs and create one long track then something like Audacitywould work ... though I'm sure why this would be a useful thing to do!
Of course, if you simply want to import two or more CDs into iTunes and have the results recognized as one album, that is supported - when you import, just make sure that each disc has identical values for Album Artist (or Artist if Album Artist is blank) and Album, and that disc numbers are set to 1 of n, 2 of n, etc. You can also join tracks from one CD during iTunes' import process - as long as the tracks are sorted in ascending track order and either all tracks or a series of consecutive tracks is selected, the Join Tracks option will be available in the Options dropdown.