In gmax, simply use the mirror modifier. It will flip vertices and faces over the plane of your choice. It will not flip normals.
So basically, if you want to create a whole body out of a half a body, clone the half you have and mirror it over a plane (usually X) at a position equal to the edge closest to your chosen axis (usually the center of the sternum). If your half model is butted up against x=0, and all points do not cross into x<0, then your mirror should match up with the points on x=0 and you can easily weld them.
If you are using something other than gmax, and face normals are flipped, find a modifier which flips them back. Gmax has a face normals section right on the mesh rollout, or a modifier which does the same. To set the faces to the opposite, choose "flip", otherwise choose "unify", followed by "flip" if needed.
In Gmax, after you flip the object, you can select the first, and then choose to "attach" a mesh, then simply select the cloned part. It will then take the cloned section (as is with any modifiers on it) and add it as an element into the original. This will retain the rotation and position of the first object, not the clone.
If you need clarification on this, or pictures, I can do that, just ask.