"To sign a goalkeeper" means to officially choose and hire a goalkeeper for a sports team, typically by agreeing on a contract and adding them to the team roster. Full definition
Avoid signing a goalkeeper — a new pair of gloves fails to improve a leaky defence 70 % of the time. (justarsenal.com)
River Plate were certainly convinced and signed the goalkeeper at the start of the 1973 season. (goalkeepersaredifferent.com)
Wenger only signed goalkeeper Petr Cech last summer before adding midfielder Mohamed Elneny in January despite many observes feeling the Gunners need more world class signings. (dailystar.co.uk)