you can with the premium version of google apps - its the split domain strategy
you would setup your mx records to point at google apps and get it to forward to your office server for accounts that do not exist on google apps
you would have to make sure your local server configured to send email for the user accounts hosted on google apps are sent there rather than simply fail (or deliver locally which means you would never receive them)
if you dont have exchange i would recommend a full switch to google apps anyway as you can gain some exchange like features such as syncing contacts and calendars with your smart phone and the fact that you dont have to manage/maintain a server and are free from worries like ZESA etc it also simplifies things because you dont need to worry about the split delivery (which from experience is really only a tool to be used temporarily while migrating accounts to google apps)
