Can I modify lightmap by hand?

I'm making a scene for mobile using the T4M plugin, and it only support 4 textures. I want to have more textures. So, I split the mesh. Then each mesh will have 4 textures to use. But, the side effect is the crossing area will have heavy ao effect...

I want to erase this ao effect.

I've heard that the lightmap can be modified by hand (maybe in photoshop with some plugin? ). Is it possible?

2111896--138548--scene.jpg

the light maps are simple .EXR files.
so any image editing software that supports the format should do the trick.