Midfielder Walker's loan has been extended to the end of the season.
Walker has scored two goals in his initial month with the Millers, the second being a vital winner against promotion rivals Bury.
Speaking on the Official Site Josh said: "From the first minute I have been here the lads have really helped me settle in. It is a great feeling being back in the automatic promotion places, where now hoping to push on and I am grateful at still being here to help the lads do just that.
"I wanted to come here, play some games and to get on the score sheet has been a bonus. It doesn't matter who gets the goals from now until the end of the season, as long as we are in League One."