In ArcGIS Engine, an array of geotransformations could be specified for MapBean and MapServer. Those geotransformations are needed for bringing layers of different spatial references into the map spatial reference. Comparable API's for manipulating an array of geotransformations don't seem to be available in ArcGIS Runtime for JMap.
Not sure if the supported geotransformations for ArcGIS Runtime is published on the ArcGIS Resources as well.
Not sure if geotransformation could be specified in well-known text. If not, how could custom geotransformation be specified in ArcGIS Runtime.