Likely it would be easier to use a Bluetooth keyboard to begin with.
There might be easier methods, but one approach may be to terminate the USB HID protocol to extract the key board data. Then emulate a Bluetooth keyboard in order to complete the connection between the USB keyboard and Bluetooth device.
It would help to know that there are several different underlying protocols in both USB and Bluetooth. Each is designed for a specific purposes.
There are several USB protocols to handle the many USB Device Classes. For example, USB uses a different protocol for a keyboard (USB HID) then for a Thumb Drive (USB MSC). Therefore there is no one generic solution to terminating a USB connection. This is why most low powered embedded devices can only terminate a specific USB device. For example a printer may only accept a Thumb Drive or a camera that looks like a Thumb Drive but nothing else.
Similarly, Bluetooth has a number of different underlying protocols. One for two channel stereo music with high latency. Another for two way phone conversations with low latency. And many more. Again, it would be difficult to create a generic Bluetooth interface for all situations.