Using Environment variables

How do I use an environment variable in Image import profile?
How do I use an environment variable within another environment variable? For example, lets say I create an environment variable of string type called "FileServer" and set the value to "Server01". I want to use that in another environment variable called "ReferenceSheetPath" whose actual path is going to be "\\Server01\ReferenceFiles\ReferenceSheet.xlsx". So I would like to use it as
"\\<FileServer>\ReferenceFiles\ReferenceSheet.xlsx"

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