Can I convert galvanized ducts to wood stove black ducts by coating it with high temp RustOleum paint?
24-gauge is 24-gauge.  However, the the silver type round pipe is coated with a rather inexpensive galvanizing coating.  This coating is to inhibit rust.  The galvanizing keeps oxygen and moisture away from the raw metal.These elements will discolor the metal, causing the metal to oxidize and rust, thus shortening the life of the metal.
Now for the down side of you situation.  When the galvanized coating gets too hot as in when used as a wood stove chimney vent the the galvanizing burns off and in the process produces a dangerous gas.  If this gas is inhaled the result will be sickness and even death.
Sorry you are going to have to put up the money for the black stuff. 
Top coating this with paint no matter the quality is not enough to ensure protection against this off gassing.