Look for a package that contains your distro's mkinitrd command. It is actually a script, and it tends to be customized for each distro.
Creating an initrd file is the last step in building a kernel, so I'm sure your distro should have it.
On my system, the initrd-<version>.gz is a compressed cpio archive. On others it is a compressed ext2 filesystem image. You can extract the files from an initrd image with "cpio -vid < /path/to/initrd". After doing that you can modify the files you want and then use cpio and then gz to recreate it.
It would be better, however, to locate the packages you need. Look for a howto on compiling a kernel. It should say which packages you need, which should include one needed to create an initrd file.