How to create a global robots.txt that gets appended to each domain's own robots.txt on Apache?

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http://serverfault.com – I know can create ONE robots.txt file for all domains on an Apache server*, but I want to append to each domain's (if pre-existing) robots.txt. I want some general rules in place for all domains, but I need to allow different domains to have their unique rules. Is there a way to accomplish this? (*In my case Apache 2.2.x) (HowTos)