Currently I'm implementing 802.11k. I would like to convert RCPI and RSNI to RSSI values.

Is that possible?

Are you still looking at that? I think there may be some validity to treating RCPI as an RSSI proxy as they are AIUI measuring the same thing (received power), but at different parts of the frame exchange.

RSNI is less interchangeable in that it is the difference of "expected" and "other" signals.

If you're trying to work out an overall "usability" score you can probably infer it from just one of the values, but with some difficulty for comparison if you had just RCPI for one and RSNI for another.