Yes, you can fit a float valve into a Waterhog. However the actual cap from an Aquaroll may not fit a Waterhog.
The float valve is fitted through a hole in the cap so, you take your existing cap or buy another and using the correct size hole-cutter make a hole in the cap and fit the valve through it.
You can buy the float valves online but check around because prices vary considerably for exactly the same thing. I bought my last one from eBay for less than £10.
Unless you know for certain that the Aquaroll cap will fit your Waterhog, don't buy the dedicated Aquaroll kit which includes the cap, just buy the float valve itself. It will come with all the necessary nuts and sealing washers.