I think I'm getting an idea of how it works... (but I could be wrong!
)
So, if you undo the screws holding the bottom plate into the extension, can it be completely separated? If so, if the bottom plate is a bigger diameter than the pole (and possibly even if it isn't) you could weld the bottom plate to your pole to make the pier, or replace the bottom plate with another one that us welded to the pole (thus allowing you to sell the extension later if you want)?
Am I on the right track or still missing something?
Al.