Thanks. With the advanced boot configuration it works fine.
If somebody doesn´t want to search, here is my solution:
open /var/www and create a directory called (for example) isos
put your isos into it
go to advanced options an add your personal menu, for example
:MENU
menu
item --gap – ---------- My boot menu --------------
item Hirens Hirens Boot CD
item return go back
choose --default return --timeout 10000 target && goto ${target}
:Hirens
initrd [url]http://${fog-ip}/isos/hirens.iso[/url]
chain memdisk iso raw ||
goto MENU
:return
chain [url]http://${fog-ip}/${fog-webroot}/service/ipxe/boot.php?mac=${net0/mac}[/url] ||
prompt
goto MENU
autoboot