Hi there,

Currently, it is impossible to do that.

But you can select the Joomla User Group of the authors on which category only available for the Joomla User Group that you selected during composing in 'Select category during blog creation / edit'. http://take.ms/jDPz8V

For an example, if you select Joomla User Group A only available for Category A so only authors of Joomla User Group A can select the Category A in the composer.