The USB port is connected to a USB-serial chip. It’s currently only used to update the firmware. Note that OpenSprinkler DC is not a USB-host device (and does not run a Linux system). If you plug in a USB WiFi adapter or something like that, it’s NOT going to recognize it.
You mentioned the Bluetooth and WiFi’s “power consumption and the impact it may have on my valves.” I am quite surprised: how much power does Bluetooth or WiFi consumes, to the point that they will impact the valves?