a quick question

you can put a ball valve on the output to the tank side of the pump with out any problems. What you can't do is restrict the flow on the pump input side. That can ruin most pumps.

Hi tat2shawn - welcome to AC!

Installing a valve after the pump outlet is no problem...lots of people, including myself, have done this.

However, I'd recommend a gate valve over a ball valve. Ball valves are kinda meant to be used for cutting off water flow all together. A gate valve, on the other hand, is meant more for partially restricting water flow. Gate valves are also more finely adjustable/tuneable than ball valves.