KML – Keyhole Markup Language is a GIS format initially developed for Google Earth while TopoJSON its a extension of GeoJSON that encodes topology it uses arcs. You can convert KML to TopoJSON file using IGIS Map Tool. You can also use it for other GIS data Conversion.
Convert KML to TopoJSON
Login to IGIS Map Tool with registered email id and password or register with valid email id if you are new. From dashboard Go To SWITCH TO and click on Converter.
Next screen of converter with three steps. Step one is to upload your gis data file. 
Step two select the desired output and Coordinate Reference System. And in the end click on Convert File button.
Next screen will be map screen. TopoJSON file is published on the map. Download converted file from download button in action column.
Check out video for more clear approach :
So by using above steps you can convert KML file to TopoJSON file using IGIS Map Tool. If you face any issue in conversion then contact us or comment below.