Hi,
http://www.juniper.net/techpubs/software/screenos/screenos6.3.0/630_ce_AddressTranslation.pdf
I doubt that you can host multiple distinct private IPs behind one single IP. even if they are in the same subnet there could be load balancing i think. This is doable till some extend if you host your public IP of the different ports and map them accordingly to the different private IPs.
Thanks,
Vikas