Thank you for replying.
The Firewall is from BayWare, it has 50 user license. The reason why I am not doing as what you suggested is because the Firewall only have two ports (one in one out) and there is a lot workstations and servers connected to that firewall through the switches.
Due to the Firewall counting the IP addresses behind itself to determine how many user license had been used, I have to use the SMC router to let my extra workstations and servers to share only one IP address in order to save the licenses.
Also, the original setting is like this:
leased line -> modem -> router -> firewall -> switches -> many workstations and servers.
I wish to make it like this:
leased line -> modem -> router -> firewall -> switches -> many worstations and servers plus SMC router.