Unfortunately there is no command-line API for the new Graphical Controls and Displays in R2015a, so you cannot programmatically set its parameters.
However, you can at least add it with add_block, though. The proper path is 'simulink_hmi_blocks/Knob'. So, for example, you can do
>> add_block('simulink_hmi_blocks/Knob', 'Example/Knob')
in which 'Example.slx' in a model on your current search path and is open.
I work for MathWorks and I have forwarded this feedback to the appropriate product team. They will consider incorporating this feature in one of the future releases.