Easiest thing to do would be to put a T-joint in the return line, splitting the output into one line that goes straight back into the tank, the other thru the UV. A ball valve on the line going to the UV would allow you to control flow rate thru the UV.
UV units are of questionable value in most cases, and a UV operating only several hours a day is likely to have little, if any, impact on your tank. Adding this branch to your filter output shouldn't put any additional drag on the filter pump, so why not let the UV run 24/7 if you have to use it?
Jim