You get a FAA private certificate. You can do the paper shuffle under 61.75 to get it. You must hold a FAA PVT to apply for the FAA COMM. Then you get at least a FAA second class medical. Then you get signed off to take the written test and pass the written. You must have 20 hours flight instruction covering the items listed in 61.129. How much of your prior instruction would apply to these requirements would have to be worked out on a case by case basis. There must be at least three hours instruction and a sign off for the checkride within the two preceding months. And then you pass the checkride.
The short version is there is no short cut or "conversion" for foreign license holders looking for a FAA COMM.