Beth i know what you are trying to say since being there snooping in your sump.
You want to run a line from your RO unit to your sump VIA a float swtich ok.
You have to drill the sump and have a certain leve lof water in there , but keep in mind that you may have to raise yuor water line to to sort out the return pump so it wont run dry.
As the RO line is drilld to a float switch the water will drop and activate the RO water to the float switch and there you have it water topp off.
BAD NEWS YOU AHVE TO TAKE OUT YOUR SUMP TO GET IT DRILLED UNLESS you have a way to se up the float switch..
mike