Hi @fl4276,
Currently there is no way to directly import .json files from 3D Slicer and it is in the pipeline but not at priority right now.
Transformations can be very tricky so what I would suggest maybe you can play around with the coordinate system transformations using the Brainstorm function cs_convert as in the tutorial: https://neuroimage.usc.edu/brainstorm/CoordinateSystems#Converting_between_coordinate_systems. More details for the function can be found in the comments of the respective code.
Also there is some similar discussion that happened in the past https://neuroimage.usc.edu/forums/t/conversion-of-coordinate-system-from-scs-to-world-coordinate-when-exporting-the-mesh/33879. Maybe you can get some ideas from this.
Let us know how it goes for you.