According to ESPN NFL insider Adam Schefter, Hagan is signing a one-year deal with the Giants on Tuesday for the minimum.
Hagan was with the team until the start of the season, when the Giants released him during final cuts.
The Giants are down to three healthy receivers, starting with Steve Smith nursing a partially torn pectoral muscle. Ramses Barden was put on season-ending injured reserve on Tuesday with ligament damage and a fracture in his left ankle.
Smith likely will miss at least the next two games.
The Giants also signed defensive tackle Dwayne Hendricks to their practice squad Tuesday. Hendricks was on the team's practice squad for the last 10 weeks of the 2009 season.
Ohm Youngmisuk covers the Giants for ESPNNewYork.com. Information from The Associated Press was used in this report.