Can you spray cellulose insulation over knob and tube wiring in a attic

No... when the humidity level goes up you could create a conductive path that could result in a fire due to old aged and possibly defective wiring. (cracked or damaged insulation)

Section 394.12 of the 2008 edition of the National Electric Code states:

Uses Not Permitted
(5) Hollow spaces of walls, ceilings, and attics where such spaces are insulated by loose, rolled, or foamed-inplace insulating material that envelops the conductors.

The reason is that knob and tube wiring was designed to operate in open air. Any insulation covering K&T wiring will cause the wiring to overheat.