It should be reasonably easy. However, it may not be worth it vs other solutions like PWM case fans with Asus Suite (or maybe even in the BIOS?). Or Speedfan or something. Note that the Sentry doesn't use the CPU's own sensor to monitor the temperature, it has a probe that you need to tape to/near the CPU. So it's temp reading for the CPU won't be very accurate.
You'll want to check how the fans (especially the two that aren't being controlled) are plugged in. Are they plugged into the mobo, or into a cable/hub that's basically plugged right into the PSU? Follow the cables from the fans
hard to see it looks like the rear one go`s to the mobo the top one the Corsair H100i V2 Hydro Cooler w/ PCS Liquid might go to a cable/hub and the twin front fan go`s into a cable/hub
In that case it may be that you can control the case fans at the rear that are plugged into the mobo, while the two front two fans are plugged into the PSU. If your motherboard has spare chassis fans headers I'd look to plugging them in there, in your position.