The 'set animation timescale' trigger can handle this, or an actor message to the unit's actor.
Cyber Jorts
In the cutscene editor it is very easy. Open up the property timeline, create a keyframe in the animation where you want to pause and scale the animation speed from that point on to .000001 or something tiny. It's not technically frozen, but it's slow enough no one will notice.
Bilxor Regular ShmoePretty self-explanatory. Say I want to freeze an ultralisk right as it is rearing back to attack like it's literally frozen in time.