You are correct, Its a part of the rad..
Now, the threaded end - does that go into the cooler? It is possible to get a female to male connector that would allow you access and connect the two both in milde steel and in brass.. but it would come from an industrial piping company like Plumbcentre, Pipline supplies, BSS, Wolsley something along those lines and it may be a ballache as you don't have an account, failing that cash only.
I had this many years ago, broke the expansion pipe clean off the top of the radiator. It was a brand new radiator too.. had a mate bore out a bolt, tapped the hole and the broken piece, loctited the thread and screwed it into the rad.. better than before and never let me down..