Add LTM to existing HA pair
dcarterjr I believe the best route would be to create the new traffic group on the existing pair first to confirm it functions. After that you should be able to add in the other devices to the HA pair the same as any other F5 HA setup. This article might help you with your configuration.
The only concern I would have is latency between the 4 devices because those other devices are at a different location. As long as you can extend your subnets for the existing pair to the other location for the two new devices it should be possible to configure this setup.