First of all it must be firmly understood that “Frost Proof” is not “Freeze Proof”.
Frost proof hydrants will withstand subfreezing temperatures for a short period of time such as climates where they get the occasional overnight freeze but they will not withstand sub-freezing temperatures for a long duration and they definitely will not withstand the extended sub-zero temperatures of the Minnesota winters.
The only truly reliable method of preventing the line from freezing in your climate is to install a line stop valve at least 3’ inside the structure to allow draining the standing water out of the last section of pipe. The problem here is that installing such a valve 3’ inside the structure would require leaving an access in your finished ceiling however there is a simple alternative.
You presently have galvanized iron pipe, which requires NPT (National Pipe Taper) threaded joints. While the pipe could be cut and rethreaded at any convenient point pipe threading equipment is relatively expensive and generally not readily available to the homeowner. You could rent a set of pipe threading dies but the simpler solution would be to cut the pipe then unscrew the threaded joints back to a convenient location in the adjacent room. You will need two pipe wrenches to unscrew the pipe joints. One wrench is used to hold the existing pipe steady while the second wrench is used to turn the section you desire to turn.
In order to prevent electrolysis corrosion the Plumbing Codes expressly prohibit directly coupling copper to iron pipe or vessels. To make the transition from iron pipe to copper pipe we are required to use a “Dielectric Nipple’, “Dielectric Union” or a 6” hardened bronze nipple.
Begin by turning your water off.
Next locate a joint in the adjacent room where it would be convenient to make the transition from iron pipe to copper and cut the pipe on the upstream side of the joint leaving a short stub (6” to 12”) of the pipe extending out of the coupling. Hold the coupling firm with one wrench while you unscrew the stub with a second wrench. You can then remove the pipe from this point to the outside faucet, or you could elect to just abandon it in place, whichever is convenient and desirable.
Select a 6” hardened bronze pipe nipple and apply some pipe dope to the male threads on one end of he nipple. Now screw the nipple into the coupling and tighten it firmly in place. Next apply some pipe dope to the male threads on the opposite end of the nipple and screw a “line stop & waste ball valve” on the nipple. A line stop & waste ball valve appears like an ordinary ball valve except it has a small bleed cap to permit draining excess water from the upstream side of the valve. Be careful when installing these valves because they are directional. If you put it in the wrong way the bleed port would be on the pressure side of the valve when it’s turned off. Look for the directional arrow on the body of the valve. The arrow should point towards the outside faucet end of the line.
Once you have the valve installed you can then solder a “male thread adapter” on the end of a piece of copper pipe, then apply pipe dope on the threads and screw the copper into the output of the valve. Continue running copper until you reach the outdoor end where you can install a common “Sillcock valve” The last 4 or 5’ of the copper pipe should be pitched downward at about ¼” per foot toward the outside.
With this configuration you will be able to turn the water off in the adjacent room, then open the outside faucet to drain the residual water out of the line. Opening the waste port on the stop * waste valve will allow air to enter the pipe to vent it thus aiding in draining the line. During winter months the outside faucet should be left open. This would allow any water that may seep past the stop & waste valve to drain out thus preventing the line from freezing.