Restricting the User list shown in Private message

Hi there

I really want to buy this plugin but need an additional feature- 

I want to be able to control which users are shown by the 'User role'. For example, I have two User roles:

1. Agent

2. Customer

I only want users in 'Agent' role to be able to lookup users in 'Customer' role in Private messages, and I only want 'Customer' users to be able to lookup 'Agent' users. 

How can I make this possible?

Many thanks

Alex