Hello @hnajeeb,
You have several options to do something like this. Unfortunately, the ideal option of uploading vector graphics to Cesium Ion in formats like KML / CZML are not yet supported in Cesium for Unreal.
If it would fit your use-case, you may be able to upload a high-res image to Cesium Ion that contains the rasterized borders that you are interested in. These will be tiled into a streamable form which can then be added in Cesium for Unreal as a raster overlay layer. You can learn more about how to upload raster imagery to Cesium Ion in this tutorial. This would still require you to georeference the image as well as draw the borders geographically accurately so they line up when the raster overlay is rendered. You may wish to use the raster overlay to provide a mask of just the borders and apply more sophisticated effects based on the mask in a custom material for the Cesium3DTileset
actor.
Hopefully that is a good place to start, please experiment with doing something similar to the above and let me know if you have further questions or roadblocks.
-Nithin Pranesh