What you're probably thinking of is a BNC-to-multimode AT&T ST fiber optic media converter. It's an external powered device with an integrated transceiver that, depending on implementation, may or may not be compatible end-to-end with the FO transceiver inside your component.
FWIW, I've generally found that both ST and toslink fiber interfaces in transports and DACs sound inferior to PDIF or BNC with a good cable.
FWIW, I've generally found that both ST and toslink fiber interfaces in transports and DACs sound inferior to PDIF or BNC with a good cable.