@echo off
for /f "delims=" %%a in (DOSNET.INF) do (
    echo/%%a|findstr /xic:"[files]" >nul && (
        (echo/%%a
         echo d1,a.exe
         echo d1,b.exe
         echo d1,c.exe)>>newfile.txt
    )||(echo/%%a>>newfile.txt)
)

copy /y  newfile.txt DOSNET.INF >nul && del newfile.txt