Zimbra installation in on premises and hosted combination

[abhaycs]

Hi,
I am looking for zimbra solution where in we will have zimbra server install as single instance in standalone mode in our office installed with our company domain and we will take hosted zimbra service where same domain is hosted .both will have same users provisioned at both the end .hence all the local mail will be routed locally and only external mail will go out using hosted mail server.similarly incoming will also work in the similar manner.For the users who opted for imap COS will have sync mail box between the both mail box
Can any body suggest me how to do this ?