How do I report the exchange of one cryptocurrency for another (e.g. Bitcoin for Ethereum) in Turbotax? I only see the option to denominate transactions in USD.

An exchange is going to result in a sale.  You are going to sell the original currency to purchase the new one.

 

This will be reported similar to a stock trade.

 

Here's where you enter or import your 1099-B in TurboTax:

Income & Expenses

1. Scroll to Stocks, Mutual Funds, Bonds, Other
2. Answer Yes to Did you sell stocks, mutual funds, bonds, or other investments in 2020?
   • If you see Here's the info we have for these investment sales, select Add More Sales.
3. Answer Yes to Did you get a 1099-B or brokerage statement for these sales?
4. At Let's get your tax info, choose how you want to enter your 1099-B (import or type it in yourself).
5. Follow the instructions.

After entering or importing your 1099 B be sure to review the detail.  From time to time your broker does not know what your basis was (amount you paid for the investment), and it will be blank.  This will result in owing excess taxes.  You can be manually enter a correction.