Or, create another initrd with the files you want and add it to the INITRD list, basically the same as a mydata.tgz file. Here is an example for a syslog server:
DEFAULT pxe
LABEL pxe
KERNEL pxelinux.cfg/default-boot/vmlinuz64-11
INITRD pxelinux.cfg/default-boot/rootfs64-11-1.gz,pxelinux.cfg/default-boot/modules64-11.gz,pxelinux.cfg/default-boot/my-syslog-data.gz
APPEND host=syslog-vm printk.time=1 lang=en_US.UTF-8 cron syslog quiet noswap nozswap nodhcp tce=/ httplist=192.168.3.4:/syslog.lst udev.children-max=24