Is it possible to export a lightmap to tweak it in Photoshop?
When you bakes the LightMap, unity automatically creates a folder with the same name of the scene file, just open the .exr files inside this folder. For example, if you've created a scene called "Scene1", after the lightmap bake, open up the "Scene1" folder, and inside it you'll find the lightmaps, that are free to be tweaked in photoshop.
Seems to me that the light baker makes a folder containing .exr files, why I don't know, but I think there are some plugins you can get that read/write those for Photoshop (and programs that can deal with Photoshop plugins too).
I'm not sure of a way to do it in Photoshop, but there are external tools that can create custom lightmaps:
Unity's Lightmapping in Maya Tutoral
There is also a way to do it in Max, but I can't seem to find the documentation.
Henrique all I can find in that folder is the .exr How do I open this inside this folder?
just doubleclick on it to open... you must to have installed photoshop cs in order to edit exr files... unfortunately i cant found any free tool to edit exr files...