The simple answer is, I don't know anymore how to add another voice, but I did once added voices in different languages on Onyx M96 with the help of Booxtor. The experience was however disappointing. The voice is very monotonous, and it cares little for the punctuation signs. The lack of inflection is killing the joy of letting a voice reading to you. My effort was certainly not rewarded.
If you bought your Max2 from Booxtor, you can ask him for help. If not, just ask Onyx. Open a thread on their forum:
http://bbs.onyx-international.com, and there is a good chance they would answer.