I have not put that up somewhere right now.. but did that last week for a RHEL 7 installation but should be the same for CentOS7. Steps are as following:
get the ISO file of CentOS 7
mount -o loop CentOS-7.0*.iso /mnt/
mkdir -p /CentOS-7-ISO-respin/{CentOS-7-unpacked,CentOS-7-iso}
rsync -avz /mnt/ /CentOS-7-ISO-respin/RHEL-7-unpacked
put your kickstart file into /CentOS-7-ISO-respin/RHEL-7-unpacked
add a new line to /CentOS-7-ISO-respin/RHEL-7-unpacked/isolinux/isolinux.cfg (this way you can use the ISO also from a DVD) label linux_ks menu label ^Install CentOS 7 with Kickstart kernel vmlinuz append initrd=initrd.img inst.stage2=hd:sdb2:/ ks=hd:sdb1:/ks.cfg
create the ISO with: /usr/bin/genisoimage -untranslated-filenames -volid 'CentOS-7.0-KS-x86_64' -J -joliet-long -rational-rock -translation-table -input-charset utf-8 -x ./lost+found -b isolinux/isolinux.bin -c isolinux/boot.cat -no-emul-boot -boot-load-size 4 -boot-info-table -eltorito-alt-boot -e images/efiboot.img -no-emul-boot -o /CentOS-7-ISO-respin/CentOS-7-KS-iso/CentOS-7.0-KS-x86_64-DVD.iso -T /CentOS-7-ISO-respin/CentOS-7-unpacked/ ; isohybrid -u /CentOS-7-ISO-respin/CentOS-7-KS-iso/CentOS-7.0-KS-x86_64-DVD.iso
dd if=/CentOS-7-ISO-respin/CentOS-7-CSB-iso/CentOS-7.0-KS-x86_64-DVD.iso of=/dev/XXX (where XXX is the Device Name of your USB drive without the partition number e.g. /dev/sdb) (Careful, that command destroys all data on the Stick)