APF and BFD (BFD needs APF to work) runs completly independent from ISPConfig. You may install it without worrying about breaking IPFC. You just have to turn off the firewall option in ISPC Control Panel before installing APF and BFD. Please do read the MAN pages and look at example config files so that you dont lock youself out.
Shuttle XPC | Intel 865g | P4 3.2Ghz | ATI 9800 Pro
Hosts: Ubuntu 6.10 ~ XGL-Beryl SVN-Gnome | OS X 10.4.8 | WindowsXP
Virtual Appliances: Ubuntu Server 6.10 | WindowsXP | CentOS 4.4