Added weight and balance of added weight will be an issue for a so small camera/gimbal.
How looks the one of Polarpro for the Mavric, I doubt it will work that easy. This is from Phantom3 exeprience, I don't have a Mavric.
Don't expect too much out of you filters for the Spark.
I have seen them on their web site. PolarPro DJI Spark Filters-Capture Perfection with ND Filters
For me the questionable point is that they don't replace the original filter (at least like on a Phantom 3) so anything you add will be extra weight the gimbal has to compensate.
I would personally not go for that way. Spark is nice for picture where you don't need ND filters but video is good but not top and currently limited.