Lookman spent the second half of last season on loan at the Bundesliga outfit, scoring five goals and assisting three more in just 11 appearances.
The Toffees are understood to be reluctant to sell the 20-year-old and have rejected a £12m bid, but Rangnick revealed that his side remain interested.
"Bringing in three more players would be realistic," Rangnick told reporters at a press conference.
"We're still in talks over Ademola Lookman - we want him back and he wants to come."
Lookman has made just 24 senior appearances for Everton since joining the club in January 2017.