Just a thought but does the manufacturer do an alternative tailstock to take a chuck? I think you will have problems however you try and do this as the tailstock doesn't appear to run on any sort of machined surface, so setting it up straight for use with a chuck would be a pain. I would forget about tapers unless there is one there already, getting an MT taper machined into it would cost a lot, even if there's enough meat to allow for it. Like I say, cut the end of the threaded shaft and make an adaptor for that thread to a threaded chuck. You would need a second adaptor to suit your existing rotating centre, so you can still use that. To make the two adaptors would be simple and cheap. Unfortunately I suspect you're going to find that the machine isn't going to be accurate enough to do this anyway, clearly not what it was designed for. First thing I would do is assess how much play there is in that tailstock screw. Whatever play there is, that is how inaccurate any hope you still will be.