How can you add a water cut off valve for a toilet to a PVC pipe?

First of all, shut your water off. Cut the PVC water pipe and connect both ends using glue. You might have to cut the water line if it's too long. Insert the valve on both ends in the middle. Turn the water back on. Be sure to check for leaks.

Usually toilet has an inlet pipe from wall to bottom of tank. If it does not have a cut off there, cut off water, drain tank, and remove inlet hose. Take with you to get the right size cut off from hardware.

Home Depot plumbing department get a self tapping shutoff valve

PVC is not code for potable water connections only drains. CPVC is code.

Try using "shark bite" shut off valve. You cut the pipe, measure the distance between the top and bottom of valve, slide "shark bite" to each end. It will require a special plastic tool to remove, worth every penny! We have used them during every rehab plumbing project. Great product and easy to do.