Hey Jeremiah,
this has been a topic many times here. Search for “Universal Z axis” and “Z axis assembly” in this forum. If you simply add this Z axis, you will notice that your spindle center will be very far away from the Z rails and the leverage force will increase. The original Z assembly has the spindle axis very close to the rails. But it has a big disadvantage: The original Onefinity Z assemblies are unable to protrude beyond lower end of Z rails. (unlike on “usual” CNC machines).
Tom @TMToronto built a Z assembly, by the way.