After this operation was canceled, the now renamed Stier was modified into an auxiliary cruiser in April 1941, first at the Wilton shipyard Rotterdam and later at Oderwerke, Stettin, and Kriegsmarinewerft, in Gotenhafen (Gdynia).

On 27 September 1942, she engaged and sank American cargo ship SS Stephen Hopkins, whose resistance inflicted so substantial damage that Stier had to be scuttled by her crew.

On 27 September 1942 Stier encountered the American Liberty ship Stephen Hopkins en route from Cape Town to Paramaribo.

Stephen Hopkins's commander, Captain Paul Buck, was posthumously awarded the Merchant Marine Distinguished Service Medal for his actions.

So was United States Merchant Marine Academy cadet Edwin Joseph O'Hara, who single-handedly fired the last shots from the ship's 4-inch gun.