Quantum key distribution with multimode subcarrier wave encoding is propitious for being robust against environmental disturbance. For application in long-distance quantum communications, this encoding has to be compatible with entanglement-assisted quantum repeaters that are commonly designed to work with dual-rail encodings. We propose and demonstrate an interface between subcarrier wave and dual-rail encodings with a fidelity of $0.95\ifmmode\pm\else\textpm\fi{}0.03$ using a linear optical circuit. The developed scheme opens the way for future heterogeneous quantum networks that combine subcarrier wave quantum key distribution with trusted and nontrusted nodes.