I was thinking that Postfix might have a #include directive, but found out in their FAQ this isn't the case
What is the most automated way possible to do this.
8. Is there some kind of 'include' directive for main.cf?
No. Most administrators with complex configurations create a Makefile that will cat the necessary files together. If you have other regular administrative tasks, add them to your Makefile too. Your Makefile can have an entry something like this:
main.cf: file1 file2 file3
cat file1 file2 file3 > main.cf.new
mv main.cf.new main.cf
Then type make main.cf to rebuild your configuration file.
Ben Franklin quote:
"Those who would give up Essential Liberty to purchase a little Temporary Safety, deserve neither Liberty nor Safety."