Can i install a water pump to an overhead tank having a single piping system?
Is the tank a pressure tank or a non-pressure tank? If it is a pressure tank, you could move the tank from the roof to the supply line just after a newly installed  pump. Add a pressure switch and a Domestic PRV. Location of the parts would be as follows:  1) supply line; 2) domestic water pump; 3) pressure gauge; 4) pressure shut off switch; 5) pressure tank; 6) domestic PRV; 7) domestic water line. The pressure switch would be set at approximately 50/60 PSIG. to cycle the pump and the PRV could be set at 40/50 PSIG.  Note; this is the same set up as a well system. The PRV could be eliminated if you desire. Also the pump should be for domestic use (bronze or stainless steel). If the tank is a non-pressure tank, adding a pump would be difficult. I hope this helps. If the tank is a non-pressurized type, you could , if you had a separate fill line, add a pump, small expansion tank,  and pressure switch to the delivery line, set the pressure switch for the desired line pressure. The pump would maintain a constant pressure on the supply line of say 40 to 60 PSIG or whatever pressure you desire.