To the best of my (limited) knowledge:
Well.. even if you attach them, you have to subclass the xxxTank class and of course it's factory.
Create the model you want.. might be an idea to make the model of both skirts 1 single model.
Then attach it to whatever bone you need to attach it to (or maybe you can just do it from X/Y coords..?)
Doing it as a whole new model would be the best performing.. since your LOD models can be alot less detailed (since you won't see anything the skirt is covering up, and the skirt itself is pretty simple.. model wise.
..
Now if you were constructing a skirted tank to be accurate.. that'd be a bit more work.. since now you have specific hit zones that specific AT weaponary doesn't penetration..
If you go there, you can probably learn a thing or two from the Destroyable Static Mesh code.