Unlike Data CD's, there is no Multisession standard for Audio CD's, so you can not create a Multisession audio CD.
However, as long as you do not choose 'Finalize' or 'Disc-At-Once' options when you first write to the Audio CD, you can add sessions to the Audio CD. For example: You can write 5 songs in one session, and then come back the next day or next year and add 3 songs in another session. You can do this till the audio CD is full, or until you choose the 'Finalize' or 'Disc-At-Once' option when you write to the CD last.
The one draw back to this is your Car and Home Stereo can only read the very first session on the audio CD and those songs contained in that session. Using the above example, this means you would only be able to see and play the first 5 songs ever written to the CD. But if you go to your Computer, you will be able to see all the sessions and so be able to see and play all the songs on the CD.
Important Note: Some Recorders, for example Yamaha Recorders, support a feature called 'RW Edit Disc'. If you have Nero Burning ROM, not supported with Nero Express, and you created an audio CD in 'Track-At-Once' mode, you can open the session and remove audio files from the first session, or add audio files to the first session. In this fashion, your car or home stereo will be able to see and play all the songs written to CD. This only works with CDRW media.