Importing Lightmaps from External Program

The 3D artists I am working with would like to bake their lightmaps in an external tool (maya) and import them into Unity 5. SO i have been going through the documentation and if I understand it correctly all you do is:

1. Import in the exr file

2. Add it to the lightmap array

3. Assign the objects render.lightmap to the lightmap array index

However, I cannot get this to work. I am confused by the differences between LightmapData and Lightmap. Pretty much I am stuck on this whole process, any help would be great. Thanks