There was a tool called WarForger that could perform subdividion of a mesh or part of a mesh. I do not know if it's still on WC3C where I found it. Basically you select a mesh, part of one or even one face, click on a button called, I think, "divide edge" and the mesh is subdivided. However, I'm not sure how the wrap and the normals turn out, but that is not a problem with most Warcraft 3 editing tools. Also, the newly created vertices lie in the plane of the triangle which contains them.
EDIT: I found the program. It does work, but do not make the mistake to select a whole mesh and hit Divide Edge. Then it tends to divide symmetrical parts of the mesh ina different way, which is... ugly and may cause problems.