Dynamically add packages on the server

I began using PelicanHPC to build a cluster system, and successfully can netboot the system.
Now, what I would like to do is to dynamically add debian packages, i.e. without rebooting the clusters. I tried
     aptitude install packagename
by chroot-ing to the $ARCH/frontend/chroot directory in the server. However, clusters cannot see the newly installed packages. How should I configure the package, so as to the clusters can dynamically see the changes in the server export directory? I suppose it is something to do with the configuration of aufs, but I could not figure out how root directory of cluster is configured.