There are different kinds. I put them on my brand new 1976 Plymouth Volare and about a week later parked next to a high curb. The kind I put on was black plastic and had a white center that rested against the center stem of the Schrader valve, so you could put an air hose on it and it would fill the tire. The white center contacted the curb and depressed the stem of the Schrader valve, and when I returned to the car the tire was flat. And to top it off, back then there was a severe rubber shortage so the car did not come with a spare tire. The ones I used did not have threads on the end for a cap.
So it the extension you have has a center that all you have to do is press on it, and that center is exposed, I would not use it.