This would probably be difficult. Google Voice can't know if it's a person or a fax machine calling until you pick up the phone and start talking, so if it offered this feature, you'd have to deal with your phone ringing whenever you were receiving a fax. (I suppose Google Voice could answer "Hello?", make the decision, and then ring your phone if the person at the other end started speaking in a non-fax-machine voice. But that would also be a shitty UX.)
HelloFax mentions in a post above that they can already receive faxes to your Google Drive, so the only compromise is that you have to have a separate phone number and fax number. Which is what most people do anyway.
Google voice already picks up the phone for you, prompts the caller for their name, then rings your actual phone. Adding fax detection in there doesn't seem like it would be that hard to do.
Or, they can answer the phone, listen for any fax modems, then continue to play their own ring sound.
