Representatives of the controversial forward, 24, contacted Warnock to see if he could train with the Millers this week, with the veteran boss agreeing to the request.
He will now be assessed in training by Warnock and his coaching staff.
Warnock said: “Someone just asked me could I have have him in for a few days and I never say no to a pro.
“He’s had his problems everywhere, hasn’t he. But I have never said no.
“He’s a couple of weeks away, so he will not be involved. But if he wants to train with us, it is not costing us anything. If I can help him in any way, I will do.
“I have never considered anything playing (contract) side. But like everything else, when you see him through the gatepost, it’s publicity isn’t it.
“But he won’t be involved for a few weeks with us as he isn’t fit enough.”
Ranger, who has previously played for Sheffield Wednesday and Barnsley on loan, has not featured in league football since 2014-15, with his last appearance ironically coming for former club Blackpool in a 1-1 draw at the New York Stadium in November 2014.
Ranger was cleared of a rape offence in 2014 and has found himself at the centre of many other high-profile controversies.