We will make simpliest as possible setup package wrapping existing installer, InnoSetup for example.
First we need make directory tree, required element is TARGETDIR Directory
<Directory Id='TARGETDIR' Name='SourceDir'>Inside we will describe temporary folder for wrapped package
<!-- temp folder -->
<Directory Id='TempFolder'>
   <Component Id='my_setup' 
        Guid='{YOUR-GUID-HERE-8148-2F82D9E7B4AE}'>
        <File Id="mysetup_exe" Source="mysetup.exe" /> 
   </Component>
</Directory>