You mean these two paragraphs
mkinitrd creates an initial image used by the kernel for preloading the
block device modules (such as IDE, SCSI or RAID) which are needed to
access the root filesystem. mkinitrd automatically loads filesystem
modules (such as ext3 and jbd), IDE modules, all scsi_hostadapter
entries in /etc/modprobe.conf, and raid modules if the system’s root
partition is on raid, which makes it simple to build and use kernels
using modular device drivers.
Any module options specified in /etc/modprobe.conf are passed to the
modules as they are loaded by the initial ramdisk.
I do not see any point how this relates to my question. I know what mkinitrd is but I am asking whether there is a way how to force mkinitrd to create ramdisk as if root were located on iSCSI device. I'm preparing initrd ramdisk for diskless boot and thought thath mkinitrd could be used as it has the ability to do it.