Koderz, thank you for the info.
I had a look into the “Third Party Library” library plugin, but this is Windows only and for dynamic libraries.
What I need to do is adding static libraries to my plugins or as a 2nd choice at least add unmodified sources files.
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Would be great to have an option in the Unreal Build Tool, that can use a set of files without Unreal PCH.
At least, when I add files OUTSIDE the source folder, the “Generate XCode project” should NOT REMOVE these files from XCode.
As Koderz wrote, the build tool doesn’t need XCode, so leave my files alone. 
Thank you for looking into this issue.