Fw: multiple ip on external interface



I have 16 public ip on the external interface.  I want some users from my local 
network to use another ip than the default on the external interface when they 
try to connect to particular websites on the internet. I want to do this based 
on username.I want to do this because I have access to some websites on the 
internet based on ip address.
One department of my company has access to a website x based on an ip address. 
Another department has access to a website z  based on another ip address. 
Is there anything i can do ?
Thank's  

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