The units would still walk up the back instead of inside. And you wouldn't be able to control the destructible to turn it back into a unit. You're off to a good start, though.
You could have the unit cast an ability, as Valandil mentioned, but instead of replacing it with a destructible, you would remove its collision and movement and place a walkable textureless destructible with the mesh of the floor, front wall, and drawbridge for units to climb up and walk out. Don't forget to have pathing around it so that they don't pop out of the sides. You could close the sides with destructibles or model-less units.
"Another siege tool. I know that on the site is already have great Tranquils model..."
I seriously do not want to hear such bullshit, you don't have to apologies, SO what if someone made a Pink Wolf? You can make another Pink Wolf if you want obviously yours has to have some other features but same applies.
So now we have 2 siege towers, doesn't mean people can't make others and upload them here.
(Pardon my language but this is absolutely absurd, I've seen it many times)
This model is absolutely gorgeous and you did very well to upload it here, Cheers and Cheers!