We don't have a GML or GeoJSON converter to CZML yet. However, we will handle GeoJSON directly in Cesium and are working on the infrastructure for that now, which will also be used for KML.
For writing a GML/GeoJSON converter to CZML, the
czml-writer project contains .NET and Java libraries for writing CZML files. Some folks have also built a
czml writer in Python.
Let us know if you write a converter as we'll be happy to raise awareness of your work.
Patrick