I imported a 1099 (which includes margin loan interest paid). TurboTax has its own ToDo item for "Interest Expense". Do I need to re-add margin interest amounts here?

Margin interest is not included in the basis of your stock.

"If you borrow money to purchase an investment, you may qualify for a tax break. The IRS allows certain taxpayers to take a tax deduction for the interest expense on some loans using Form 4952. However, the tax ramifications of investment interest can be complicated, as the IRS only allows a deduction for certain types of investment interest. Additionally, the Alternative Minimum Tax (AMT) could completely disallow the use of your investment interest deduction."

You will have to claim the interest on FORM 4952.

In steed of using the search feature I'm going to make an alternative suggestion.

Log into TurboTax.

1. On the sidebar go to Tax Tools
2. Select Tools from the drop-down menu.
3. You will get a box with four green squares.
4. Select Tax Topics on the top left.
5. Type the area you want to go to, or the form you want to post. e.g., 4952
6. Post your information here.

Please see Investment Interest Expense Deduction for more detailed information.