Replacing geometry on a rig in Blender

[r1chard3]

I'm trying to replace geometry with a lower resolution version on an existing rig in Blender. (Maya is my native tongue, so forgive my bad Blender)

My procedure is to import the new geometry and add the object modifier Armature.

When I add the existing Armature is isn't generation the clusters for the bones.

What am I doing wrong? Is there an established procedure I should be following?

Thanks