The way I do it is to periodically move the unit every .03 seconds. I turn off collision and check pathability every time i move it to make sure the point it is travelling to is walkable. If not I stop it and turn collision back on. There is also some spells in the spells section that have the unit move left or right to get around the spot that is unwalkable. When the spell is over you turn collision back on. This allows you to go through any units you want to. In my case I have it ignore friendly units and detect when it comes within range of an enemy unit and deal damage to it.
All that being said though, it doesn't really help you continuously move through units as you walk around and cast spells. So I guess you should disregard my reply