How to change source port in MASQUERADE mode?

[Richard Liu <richardliu@ms1.techarea.org>]

Hi,
I have a question about change tuple source port.
I have a nat device, and I want this device can change tuple source port
in some special conditions (not always).

I want change tuple source port in MASQUERADE module.

for example:
    Destination                Gateway                Source
    DA(Ipaddr):Port          GA:Port                SA:Port

In Masquerade modules
                                    src:GA:1000           src:SA:1000
                                    dst:DA:23               dst:DA:23

But I want to the src port be changed in some conditions like this.
                                    src:GA:50000+1000  src:SA:1000
                                    dst:DA:23               dst:DA:23

I am tracing source few days, but I still don't know which function
allocate tuple source port.
Does anyone can give me some hints, I am very appreciate.