You might be interested in this:
http://support.google.com/a/bin/answer.py?hl=en&answer=173534
The page discusses options available for Google Apps such as split delivery, routing and dual delivery. You would likely choose the proper method and select "provisioned accounts" only in advanced setup. The rest would be handled by your domain as secondary MX. I've not tried it so this response is based on a quick reading of that support doc. But it seems a good start to resolve your issue. Cheers.