Update 9/29/16….Since writing this post there are now ways added to the game where you can technically use in-game cash to “buy” donuts. Check out this post to learn more about XP and Donut Farming.
There’s no clear cut answer to that only theories but TSTO is a “freemium” game, which means that you can play the game for free and still have it be very enjoyable but there are also premium content additions that can take the game to another level. And if you want that premium content you have to pay for it.
It’s also doubtful that there will ever be an option to turn game money into donuts. EA is a business, businesses need to make money in order to pay salaries. The income from the donut purchases are what go into creating new levels and content for the game.
Keep in mind though they may still do a promotion where you can convert another type of game currency you earn into donuts, like they did last Christmas with Santa Coins.
Remember: Donut Purchases=new content