By all means experiment but the acetone will likely pull freebase with it into the water. Also, acetone is considerably less soluble in basic water - as previously mentioned - so acetone separates from sodium carbonate solution. Furthermore, acetone will decrease the solubility of sodium carbonate in water, so if the sodium carbonate solution is near saturation it will start to crystallise. At some point you might end up with four phases as I did when playing around with aqueous isopropanol, DMT freebase and sodium carbonate some years ago.
And finally, even a trace of acetone will prevent freeze precipitation because of the extreme solubility of FB in acetone that you mention.
As a footnote, even quite dilute solutions of acetone can be surprisingly flammable.