you could either give them the 'astral' ability (somewhere within neutral ones) or change their color to '50%' transparency (the trigger is found under 'cinematic' or something like that)
the first one MAKES the unit ASTRAL with all the advantages and disadvantages that come along with that (except bot beeing attackable by 'non-magical' attacks)
and the second one needs 'recoloring' every few seconds, for colorchanging effects (like ice) also alter the units transparency...