yes, you are right, for '/etc/network/interfaces', you need stubs.
copy the original stub to customizable place:
sudo mkdir -p /etc/zentyal/stubs/network
sudo cp /usr/share/zentyal/stubs/network/interfaces.mas /etc/zentyal/stubs/network/interfaces.mas
add your custom things to the '/etc/zentyal/stubs/network/interfaces.mas',
but first please post your custom '/etc/network/interfaces' content.