Hey Mike,
you did not show what you mean with bit extender. If I would ask this question, I would provide a link to the manufacturer’s page with datasheet of the device in question.
But if I imagine what it could be, it would of course be possible to use it, but it would possibly put more horizontal (radial) force on the spindle’s bearings than advisable, especially if you mill sideways and not only drill.
If you want to drill deep holes into your machine bed once, I would simply mount the Z assembly in the lowest position. If you want to work lower than the machine’s feet more often, especially if you want to mill sideways, a bit extender should be prohibitive except if you want to ruin your spindle. In this case you should put your Z travel there where you need it.
Versatile machines in the furniture industry have the possibility to raise and lower the entire X axis, as shown here, that would surely be what you want, but is is difficult to imagine to retrofit this on the Onefinity.
Many people here showed their vertical clamping, maybe you seek their advice. I think they mainly make joints on drawers.