Can steaming cans of jams be safely used instead of boiling to sterilise?
The key issue is getting the contents of each jar up to the requisite temperature for the necessary time, all the way through to the center of the jars.  It would be conceivably possible to use steaming to do so, if the requisite controlled temperatures and times could be regulated, and appropriate recipes developed.

However, according to the USDA as reported at the National Center for Food Preservation:


  Steam canners are not recommended because processing times for use
  with current models have not been adequately researched. Because steam
  canners do not heat foods in the same manner as boiling-water canners,
  their use with boiling-water process times may result in spoilage.


If purpose built steam canners are not recommended, I would strongly urge you not to improvise with other devices.

You should only use recipes and methods for canning that come from highly reputable sources, such as a University Extension.