Não use start /wait ren ...
:
for /f %%f in ('dir /b c:\source\*.txt') do (
move c:\source\%%f C:\destination\myfile.TXT
IF EXIST myfile.TXT (
start /wait dtexec /F ImportFile.dtsx
ren myfile.TXT myfile%date:~4,2%%date:~7,2%%date:~10,4%%time:~1,1%%time:~3,2%%time:~6,2%.TXT
)
move C:\destination\myfile_* C:\destination\OldFiles\
)