It depends on what you mean by reduction gear.
If you simply mean the worm on the table (i.e. in the case of a 6" vertex which is 90:1 reduction) then it's designed for that.
If you wish to reduce the gearing further by driving the input shaft to the table via a reduction gear then you can do it providing you're not reducing it by too much.

Basically it's down to the maximum number of steps for a single 360 degree rotation of the table. These can't exceed 65535 steps. So with a half stepping (400 step) motor, and a 90:1 reduction ratio the number of steps per 360 = 400 * 90 = 36,000.
If you tried to drive this through a 2:1 reducer you'd need a value of 72,000 which is too high.

Hope this helps
Steve.