Hammill was sent off in Barnsley's last encounter with Sheffield Wednesday in December.
Winnall left Barnsley to sign for Sheffield Wednesday in January.
The former Wolves man reacted badly to being substituted in the Tykes defeat.
The 28-year-old scored in a recent 4-0 win at Molineux.
Johnson, who swapped Oakwell for Ashton Gate in February, is said to be courting players from his old club and Preston North End.
Barnsley's Adam Hammill is wanted by Championship sides Reading and Bristol City, a report claims.
Bristol City boss Lee Johnson reportedly wants to sign Barnsley winger Adam Hammill.
HITC Sport understands former Huddersfield Town winger Adam Hammill could be set for a move to Scotland with Partick Thistle.
HITC Sport understands that a man linked with Preston North End of late is ready to leave the John Smith's Stadium.