You could define them as a rigid body using fix rigid, and if you
assign each of them appropriate initial velocities, the body
will have an initial angular velocity (omega) which will persist.
There are options on fix rigid to keep the center of mass stationary,
and to turn off torques so that the body will just rotate freely.
Or you can wait until next week, when we're going to release a fix move
command to allow groups of atoms
to have their motion prescribed in various ways, including
a rotation option.