|Sorry, I meant to say all variations of %f, so not just|
for %%f in (*) do ren "%%f" "%~nf_%username%%~xf"
but the full treatment:
for %%f in (*) do ren "%%f" "%%~nf_%username%%%~xf"
Of course, I'd advise not to keep the batch file in the same directory or it would also get renamed.