Since some newer hardware sometimes won't work properly with the kernel from stable you like to use newer kernel versions. You can use the kernel from Grml, or use Debian's backports kernel, which are both available in the package repository of the FAI project.
1. In $FAI_ETC_DIR/NFSROOT, remove any other linux-image package and add:
PACKAGES aptitude I386 #linux-image-3.10-1-grml-486 #linux-image-3.12-0.bpo.1-486 # linux-image-3.13-0.bpo.1-486
PACKAGES aptitude AMD64 #linux-image-3.10-1-grml-amd64 #linux-image-3.12-0.bpo.1-amd64 # linux-image-3.13-0.bpo.1-amd64
2. In $FAI_ETC_DIR/apt/sources.list add
deb http://fai-project.org/download jessie koeln
3. Now you are done and can rerun fai-make-nfsroot and modify your PXE configuration accordingly.
4. You can also install these kerneles into your new system by using them in package_config/DEFAULT