John,
If the Harmony 890 can add Secondary controllers to the network, then you can set up the network the the 890, add in ThinkEssentials, and then control the devices in both places.
If the Harmony 890 can _become_ a Secondary controller, you can set up your network with ThinkEssentials, put the 890 in "join network/replication receive" mode, and then press the "Add Device" button in ThinkEssentials...
I have heard rumors that only the 890 Pro could add in Secondary controllers.
Also, one more note: with ThinkEssentials Pro, there is another way to deal with this situation. If you have the Home ID from the 890, you can fully "recover" the network with the software. You'll need to remember to only use the 890 as your Primary, and to re-recover the network on the PC if you add/remove any devices with the 890...
Chris