no but you can make a trigger that moves the tree to the position of the unit while it walks...
And if you don't want the units to move while being a tree you can always change the movespeed of the unit while it is being a tree (dunno if there's an gui action for this since I don't got trigger editor open right now) But you can replace the hero with a dummy hero that can't walk...
Will probably work
But make sure that the tree skill is based of a skill that makes the unit invisible for other units (which means that gems of true sight and such won't affect them

...) What about locust?
there are probably lots of solutions to this... You just have to find them out
EDIT: (opened world editor and saw that changing the position of a tree can be difficulty :S) But I'll try it for ya
other way to do it: export the model of the tree that you want to use for being a unit. And let someone re-skin that model... for example:
warcraft 3 viewer: file -> open mpq -> war3.mpq -> under mdx files (in the treeview) go to: doodads-> terrain -> the tree kind that you like and export the file by clicking on: current file -> extract to mdx then when your done go to blp files (also in the treeview) and then: ReplaceableTextures -> your tree you want the skin of... and again go to current file -> extract and convert (and save it as blp or bmp depending on how the person that wants to do the re-skinning of the trees for you wants to have the skin file, and maybe he needs the .mdl instead of mdx... I'm not sure about how that works yet

But there are a few programs for converting .mdx to .mdl if I'm right...)
(I don't know much about skinning :S But that might work so: import the model + skin into your map and now when you use the model, it will probably work for units too...)
I'm beaten :S Haven't found much other solutions yet....