Dynamic NAT from pool on FreeBSD

Good day,

I have a FreeBSD 8.1 server installed and connected to network. (See topology screenshot).

I need to masquarade the PC ip 192.168.1.2 on FreeBSD to 1.1.1.3, not to 1.1.1.2. On router the the source traffic must been from 1.1.1.3, not from 1.1.1.2. Is it possible?

I tried with assigning secondary ip 1.1.1.3 with alias on rc.conf, it didnt worked.