Iwelumo joined the Addicks on a free transfer in the summer and will return to Layer Road for the first time on Saturday.
The tall striker, who made his Charlton debut against Scunthorpe on the opening day of the season, scored 37 times in two successful seasons with the Us.
"Big Chris is always a handful, he scored a stack of goals for us in two years and he's started well for Charlton," said Williams.
"But we know all about him here and we'll be looking to keep him quiet."