Firewall/NAT

Hello:

Below is my config.

Public interface IP: A.B.C.D
Internal Interface IP: X.W.Y.Z

I have setup a NAT, however I found that when a request from Public "e.g. Source IP S.S.S.S". It can reach with my internal Host.

From the internal Host, it show that IP "S.S.S.S" is accessing. Is it possible to config the m0n0wall to use it own IP to access this host instead of the original Public IP?

Currently:
S.S.S.S -> A.B.C.D, after NAT S.S.S.S -> internal Host

Want change to:
S.S.S.S -> A.B.C.D, after NAT A.B.C.D -> internal Host

It is possible? if yes, can you give me the hints?

Thanks!