I think that's the whole Idea of an "integrated spindle", that is that you don't need a spindle and a motor. If you add a gearbox, you effectively use the spindle as a motor, and then the gearbox acts as a spindle.
I don't think that driving the spindle at low RPM will put extra load on the bearings. As I see it, the axial force on the spindle is a function of the chip load (but i might be wrong).
It will cause it to produce more heat, so you'll need a decent radiator, and it probably won't work with air cooled spindles.
Also make sure that your inverter have this "torque boost" feature (and that it is working).