Defining the path for multiple files

Hello,

I am starting to work on a sample project, wherein I need to include multiple C++ files, all present under a common folder.
Can I define that common file path under a variable like "app_sources", so that in CMakelists.txt I can define as: 

target_sources(app PRIVATE
  ${app_sources/sample.cpp}
  )

where can I define this path "app_sources"..?

I am using nRF Connect extension on VScode.

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