Idea:
if you change the model's sequence names from Walk to Walk Fast, and then change Walk Alternate to Walk, then set the unit editor stats for Animation - Run Speed and Animation - Walk Speed to be two different numbers, it should automatically cause the unit to change which of these animations he uses based on whether he is affected by a "slow" spell or not.
If you want an example, the Pitlord hero ingame model has these settings. (Only in the Frozen Throne tavern hero version -- the Azgalor and Mannoroth campaign heroes from the Reign of Chaos do not have this.)