He joins the Gers from French top-flight Angers on a season-long loan, subject to global clearance.
Sadiq will become Steven Gerrard's ninth summer signing and the Nigerian striker admits he was attracted to Ibrox by the former Liverpool captain.
"It is hard to say whether it will be done before the first leg [of Europa League qualifying on Thursday], but we will be doing more business".
Coulibaly follows veteran goalkeeper Allan McGregor, full-back Jon Flanagan, centre-backs Connor Goldson and Nikola Katic, midfielders Scott Arfield and Ovie Ejaria, and winger Jamie Murphy - on a permanent deal after a successful loan spell - in joining Rangers. He must be registered 24 hours before Thursday's Europa League first leg tie at home to Shkupi if he is to be available for the Gers' first competitive games of the season.
Umar, 21, spent the second half of last season on loan at Dutch club, NAC Breda and scored five times in 12 league appearances.
Coulibaly is excited to play under Gerrard but admitted he was not sure if it was a joke or not. "When they asked me if I want to join this team or not, I said yes but I thought it might not really be Gerrard!"
"Rangers is a good team, I'd heard the name before Gerrard came in here".
"He can help me to develop".