Hammill, 27, released from Huddersfield by mutual consent in September, returns for a second spell at Oakwell after signing a short-term contract.
Toney, 19, signed by Newcastle from Northampton in the summer, has agreed a 28-day loan deal with the Reds.
"We are delighted to have (them) on board. (Hammill) has a great pedigree, he's quick, direct and will get crosses into the box," Johnson told the club's official website.
"I'm sure the fans are just as excited as I am to see him back in a red shirt.
"He's turned down interest from elsewhere because he feels at home here and we're delighted to have him back."