A good metal fabricator should be able to handle that for you. There are several options that I can think of myself. If the pipe you will use is threaded, then a threaded bushing could be welded to your ring. If it isn't threaded, a slip over tube with a hole drilled for a pin could be used. Lots of options!
In another thought, its strange that the pipe holder would be missing?? Is it readily visible as to where one was previously located and possibly broken off? Or is it not a curb sign at all but a hanging sign thats still in its mounting ring??
