I have a 1/2 inch copper water pipe entering through the florr of my barn. The water source is unknown but the pipe is active(has fresh water running from it it). I presently have it capped off with a piece of hose and hose clamps. Is there a way to install a valve without turning it off? As I mentioned, I do not know the source and I have closed all water off on my property and it is still active. Any hints would be appreciated! Thanks! :confused:

It could be a spring. Use a compression fitting to attach to the pipe. From there you an go to whatever suits you, a valve, a threaded female pipe and then a valve etc.

If you wish to cap the pipe off and still retain the ability to draw water from it then a 1/2 X 1/4" Compression Angle Stop,(see image) is the way to go.
Pick one up at any hardware or big box store. Good luck, Tom