If the chart if made before festival version (or the map that not using any funky slider), then you can use MaiConverter (https://github.com/donmai-me/MaiConverter) and convert it on your own. The result may not be perfect as if the chart was changing polyrhythm a lots like PP or Our Wrenally, then the polyrhythm ratio when converted aren't perfect (as 1/7 can be 142/995); but there is a way to fix that is to change the epsilon error to be smaller/larger in the rounding/finding gcd function of that python script.
Other than that approach, only way is to chart it yourself.