How to compile initrd image adding new network drivers
 

I was looking for help in building an initrd image, which should be built adding new drivers to it.
Could someone help.

Use the mkinitrd command. See its manpage.

mkinitrd --with=your_desired_module_name <image_name> <kernel-version>

How to compile initrd image adding new network drivers
 

I am using a PXE server to install rhel3 and rhel4 with all updates on x3550 which has NetXtreme NIC card. I am trying to load the module to the initrd image, but i could not find the network module for the exact kernel. How should i make it compatible irrespective of the kernel version.

Modules must be built against each particular kernel. You can't build a module against one kernel version and use it against another.