The frequency output is a simple digital output pin so at best you might be able to get the processor to provide a pwm version of the output but that’s not readily convertible to a sine wave.
With some additional circuitry on the frequency output you could filter the square wave output and filter out the higher order frequency components. The problem is that you would need a tracking low pass filter to cover the whole frequency range, this could be achieved using a programmable active filter perhaps like a MAX262 that could track the standard frequency output. However the MAX262 would only allow the cutoff frequency of the filter to be 140KHz so not the full Nano output range even then.
There is something else to say though, as the Nano is limited to 1Msps viewing a sine wave comprising less than say 10 points would look pretty square anyway so perhaps an upper limit of 140KHz for the filter wouldn’t be so much of a problem.
I’d also like to see the full V3 source code but I understand if Benf is protective.