Please make $(SolutionDir) value accessible in Bamboo.

Our team uses multiple post build events in visual studio projects where they use the $(SolutionDir) environment variable to help define the path to the .sln. When Bamboo builds the .sln it does not capture the value of $(SolutionDir) and subsequently causes "Undefined"../Bin/Release
The "Undefined" is supposed to be the value of ($SolutionDir).
Can you please make it that all the env variables on msbuild and devenv are available in Bamboo.
Also I tried to define the ($SolutionDir) myself but you can only set alphanumeric values. Can you please make it available so that we can set any type of global variable that we want in bamboo.