Ferdinand, who was released by Manchester United at the end of last season, has signed a one-year deal and Redknapp is keen to bring in Caulker from Cardiff as well.
"We've agreed a fee for Steven Caulker, who I like very much," he told Sky Sports News.
"I had him at Tottenham as a player and he's done very well. I think for him to come and play with Rio would be a great experience for him.
"I'm looking to play with three central defenders this year. So (there are) those two and then I've got Richard Dunne, Nedum Onuoha, Clint Hill, I've got real good defenders here."
Caulker was signed by Cardiff last summer but could not help the Welsh side avoid relegation from the Premier League.
It is understood he has a release clause in his contract, which has been triggered by Cardiff's relegation to the Championship.
"I think for Steven Caulker to come in and be part of it with those experienced players will be great for him," added Redknapp.
"I think Rio will be a big influence on him. I think for Steven it will be the best education he could ever have in his life to play with Rio."