Put the install files in an NFS accessible location on the Server. Make a directory corresponding the 'nfs' directive in the install.cfg file and mirror the FreeBSD install files there, you will want it to look somewhat like this:
ABOUT.TXT TROUBLE.TXT compat20 floppies ports ERRATA.TXT UPGRADE.TXT compat21 games proflibs HARDWARE.TXT XF86336 compat22 info src INSTALL.TXT bin compat3x kern.flp LAYOUT.TXT catpages crypto manpages README.TXT cdrom.inf dict mfsroot.flp RELNOTES.TXT compat1x doc packages
Copy the compressed packages into the packages/All directory under nfs.
Make sure you have an INDEX file prepared in the packages directory. You can make your own INDEX entries like so:
alfred-1.0||/|Alfred install bootstrap||alfred@FreeBSD.org||||
Then you can install custom packages, particularly your own custom post-install package.