Yes, Na2 will cool the tone. Since you generally use less of the Na2 than you would the traditional platinum solution (potassium chloroplatinite), you won't get as noticeable of a difference in color shift, though.
What developer are you using? If you're using potassium oxalate, you could switch to ammonium citrate or sodium acetate and get a cooler tone that way.
Off topic: It's snowing here in Santa Fe again. We got 2 feet last weekend (A record) and it looks like we could get 6 inches or more tonight. Hooray for global warming!