Good news, it is possible. Basically you can tell that any email address on existing on Google Apps must be passed onto another mail server.
Our company has about 30 emails address on Google, mainly for the senior staff. This gives them the ability to access there emails on their mobile devices, as well as keeping it all in sync when using Outlook, plus it makes a handy online backup. The rest of the email addresses exist on another mail server. You then setup then setup a default route which only applies to recognized email address, and anything else is routed to another mail server. Basically, all incoming emails for your domain will hit the Google email server, and then unrecognised email addresses will be routed to the another email server.