As explained by @idefix37, this is not possible.
As an alternative, you can use Humidity map at 700hPa with a specific color above 90%. This level of humidity will indicate a potential front. You can combine it with the temperature map at 850hPa to see the air mass with different temperatures indicating fronts as well
I’m not a programmer by all means, but it sounds as if an algorithm could do what you describe here, no? 😳
I'm not sure that interpolating computed forecast values would be a good idea, as generally weather evolution is never linear. Such an artificial interpolation would get of course a more "beautiful" animation on screen, but I don't think Windy would assume the wrong interpolated values...