Fabricating objects from a player's gameplay, for example, collectibles of valuable game items, or custom game controllers shaped from game objects, expands ways to engage with digital games. Researchers currently create such integrated fabrication games from scratch, which is time-consuming and misses the potential of integrating fabrication with the myriad existing games. Integrating fabrication with the real-Time gameplay of existing games, however, is challenging without access to the source files. To address this challenge, we present a framework that uses on-screen visual content to integrate fabrication with existing digital games. To implement this framework, we built the FabO toolkit, in which (1) designers use the FabO designer interface to choose the gameplay moments for fabrication and tag the associated on-screen visual cues; (2) players then use the FabO player interface which monitors their gameplay, identifies these cues and auto-generates the fabrication files for the game objects. Results from our two user studies show that FabO supported in integrating fabrication with diverse games while augmenting players' experience. We discuss insights from our studies on choosing suitable on-screen visual content and gameplay moments for seamless integration of fabrication.