Pretty much any of the modelers out there will do what you want unless, like fazzel said, adding or subtracting verts. Also you can't move the .obj its self. You can only move the vert of that object.

Scale is not a problem. poser is smaller than most apps, so, Wings  I use Import 20.0 and export 0.05. I think Geep has a tutorial on how to morph with Wings. In Hexagon I scale import 500.0 and export 0.002. You just need to know what the scaling of the app your using.

The .mtl is saved with the .obj but you really don't need it very often.

I would suggest reading Geeps Morphs in Wings3D tutorial. doesn't matter if you don't want to use wings, it will answer a lot of the questions your asking.