how to append audio into existing ogg file

Hi,

I used example source code encoder_example.c from libvorbis-1.3.3 
library to encode audio into ogg file. Which works perfectly.
I would like to ask, how to modify encoder_example.c to be able to 
append new audio into already existing ogg audio file?
Could somebody help me, please?

Thank you very much in advance,

Regards,
Pavel