Fed does not have the authority to do that, per se. Fiscal stimulus is congress and the Treasury's job. Monetary policy is a completely different thing. The Fed is already buying Treasury bonds, which are in part being used to fund the direct stimulus measures in the form of PPP grants/loans, unemployment payments, and the $1200 check. In addition to buying PPP loans themselves. So the current situation is about as close to what you are suggesting as is legally possible.