If the stage is a .X accessory file, even MMD can do it. But in MMM, just load the accessory file, then move the slider of 'ALPHA' at the second menu from the bottom left. Value of 1 = no transparency. 0 = full transparency. Then click register frame. Then it should be enough.
If the stage is a pmd or pmx file, i would suggest using MikuMikuEffect, then use the effect known as ghost.fx. Ghost .fx exists as both a shader and an accessory file. The transparency of the accessory file determines how transparent that models with the ghost.fx shader disappear. This way is a lot crappier, but it gets the job done if you absolutely cannot use .X accessory files