Since the IO has buttons 1 thru 5 on jamma and 6 on the extra connector, you should be able to use a jnx atlas with a slight mod (i.e. 2 jumper wires from the atlas to the io ext connector) to accommodate b6 for each player.
Not commercially or as a hobbyist aftermarket product, but you can easily make one for yourself. I have done this personally. You just need a soldering iron, one of these connectors, a some JST NH pins and header with the right amount of positions, and off you go. As of late, I have kept a lot of those Hirose connectors handy to build harnesses for different boards and for MAME projects.
If you are really stuck, let me know. Might be willing to put one together for you.
The hitch here is the jst is only good for button 6. So you would have to route buttons 4 and 5 [of the cps2 connector] to the jamma edge of the sega IO. Hence why I recommended the atlas as opposed to crimping up an adapter.