Load Balancing on both external and internal interface

Hi, I'm new to Big-IP and F5, but used to work with Cisco Local Directors many years back. Anyway, we're doing a big mail migration project (and when I say big I mean - approx 18 million mailboxes). The new platform uses Big-IP 8900s. As part of the migration we want to start by putting all the traffic through the new load balancers first. However, rather than sending the traffic to the real server pool on the internal interface, we want to start by sending the traffic back to the existing mail platform (i.e. by adding pool members which are actually in a different part of the datacentre). This will involve (hopefully) load balancing the traffic back out on the external interface to the existing platform (as I said, this won't be going out onto the internet, just in the same data centre). We'll probably use Member Ratio load balancing to achieve this. Then, once all the traffic is passing through the Big-IP, we will start to increase the ratio of the pool of local servers until all traffic is on the new platform. Has anyone done anything like this before? There are a bunch of routing issues we have to solve on the existing platform which are not of concern to the Big-IP. However I'm specifically asking about receiving traffic on the external interface and load balancing back out on that interface. Is this possible? Secondly, can I then start to have some traffic going out on the internal interface (as we increase the ratio of the new servers) as well as the external interface. Neil