How do I import hundreds of S&P Index option trades from my 1099?

These are Section 1256 contracts and I don't think there's any "import" function available here.  Use the "Options and Straddles" interview.

Tom Young

Turbotax Online has problems dealing with hundreds of transactions

As an active optrions trader you must be aware that Box A and Box D transactions with no adjustments do not have to be itemized on Form 8949. Thus there is no reason to import those transactions.

instead report subtotals on schedule d Line 1a  for A and Line 8a for D

If you have wash sales, it gets more complicated since those have to be itemized on Form 8949 and the summary totals adjusted accordingly.