Well, good luck. These devices do have very low output impedances, so building a transformer over that bandwidth will be challenging.
Maybe you could do something unique and combine the output impedances in series somehow with a stackup. Lets say one device has 5 ohms output impedance. If you could design a structure to connect 5 of those outputs in series, you would have a 25 ohm impedance to match to the 50 ohm output. At these lower frequencies, maybe it could be done.