It’s either a Mesh or a Curve; if it’s a Mesh you can’t convert it to a mesh (first sentence of your first post), if it’s a Curve it’s default resolution (DefResolU) along the U vector will be in the Curve and Surface tab in F9 buttons. It doesn’t matter how it was generated or whether it was done in an external app and imported, it has to have a DefResolU to be able to be displayed.
I think you want the Remove Doubles option, and set the resolution up to how many blender units away you want the vertices to be. See the Mesh Tools button in edit mode
Modifiers can only be applied in object mode, but they can be set in edit mode. The Decimator reduces face count, and is easiest to see in action by decimating a sphere. I just tested and it reduced a sphere from 1000 faces to 100. It does convert to triangle faces though, which is bad for animating meshes.