I don’t see why not… you might have a small RFM69 network, let’s say, all sending very fast and low-power messages to a local RFM69 gateway. Now instead of the RFM69 gateway being connected to the Internet, it can be connected to a LoRaWAN Device, which will forward the messages to a LoRaWAN Gateway.
About RFM69 and RFM95 communicating to each other, as mentioned both support FSK and OOK. In terms of signal modulation they should “see” each other, but it’s important to note that those radios implement a simplified “frame format” with pre-ample, CRC and other fields, that’s probably the part which needs to be compatible for both radios to communicate.
If your idea is to forward messages from RFM69 devices to a LoRaWAN gateway, probably using a hardware with both radios would be easier than trying things to save a few $ in hardware. That’s a typical example for a Home Alarm, where you don’t need every single window sensor to be LoRaWAN, but have one or two LoRaWAN Clients working as gateway for a very low-power RF network.