No pump house, my pressure tank is in the ground. My concern is that there may be a check valve between the pump and psi tank. If I Tee in on the wrong side of the valve then it would all be for not.
If you can see the pipe from the pump to psi tank, you will see if there is a check valve there, or just add one in the main line you splice into, or at start of the new line.
Many pumps have a built in check valve and most installers will add another in the line usually between the 1st and second pipe joint, or 20'ish on poly as a safe guard.