It's always problematic to add a liquid to buttercream. In the case of champagne, the flavor is not concentrated like a liqueur, or bold like a brandy, thus is lost in translation. Extract will give you the flavor you can't achieve without excessive amounts of actual champagne. You can still add actual champagne, but add the extract first. Then add champagne a tablespoon at a time until you reach the desired flavor. Keep in mind extract also contains alcohol. Absent heat, the alcohol from both the extract and champagne will remain. Alcohol needs a temperature above 170 degrees to begin evaporating.