I suspect it's not designed/intended as a photography stand. But from looking at it, I'd start by removing the brass top. It MAY be simply screwed in. If so, the remaining threaded hole may be able to have a piece of thead rod (or cut off long bolt) screwed in that has the standard mounting bolt size (I can't remember the dimension...getting old really stinks!). However, it looks like a spring loaded popout little ball that locks in the rod, so I'm thinking there's no threads.
The top knob in the picture appears to be a vertical adjustment 'clamping' mechanism. If so, the 'spring type' ball lock would make sense and there's multiple such locks on the rod. So, I'm guessing the bottom knob could be some kind of 'normal' ball head control. In which case, unscrewing the top black part MAY expose the right size thread...or one that can be adapted to the proper size.
If all else fails, unscrew or otherwise remove the 'ball head' part. There must be some kind of thread-on method to attach the 'head' part (or parts, or assembly) to the 'base' tripod. Hopefully, IT would be the correct thread size. Then it's simply a matter of purchasing your choice of ball or pan heads for the tripod and screwing them in.
One concern I have is that of weight capacity of the tripod. If it 'easily' flexes when you press down with gentle pressure, I'd be very leery of putting several thousand dollars worth of camera gear on top of it.