Applying Mirror Modifier with Shape Keys on a rigged Model Question: 
Is there a way to quickly apply a mirror modifier on a rigged character with shape keys? 

Story:
I have a rigged game-character and use shape keys to later transit between a muscular and thin person ingame. I use this procedure for clothing as well. Since its not possible to apply the mirror modifier with shape keys the FBX export fails to export the shape keys, which makes it impossible for me to use them in my game.

After intense searching i found out that this is partly possible to do by Hand. The next problem is the Weight-Painting. Since i duplicate my low poly character, to model my trousers or shirts, I need the exact same weight painting, which will not be mirrored correctly during the process I mentioned. 
Further research explained me how to mirror the weight painting by hand. My Problem is, that i have to do a lot of Models for the game and it will too much time to do everything by hand. 

I doubt that i am the first to have this problem and hope someone resolved this already. 

Thanks!
For anyone who still wants to know (P.S. I did this in Blender 2.79):

This doesn't fix it entirely, but....

You can remove the mirror modifier,
and duplicate and then reverese the vertices onto the other side.

(E.g. move 3D cursor to centre of mesh, duplicate verts, and scale along x-axis by -1 around the 3D cursor)

Some of the shape keys might be close enough.
But some may need tweaking.

Depending on how lucky you are, this may or may not be easier than removing shape keys, applying mirror modifier, and re-do-ing all the shape keys again :/