In the Asset Flow calculator, you can roll a tax-advantaged account into a new account (like a spouse's account) at death. It must be done manually, as the calculator is designed to show what happens at liquidating.
In Asset Flow, show the principal amount distributed at death going into another IRA (or similar) account the next year with the “Tax Deductible Contributions” box checked. The tax deduction in the new account will offset the taxes paid in the old account for a net “0” tax. You will, however, have to enter it into the new account as a “Payment” at the age 1 year after the death. This is because the “Present Value” field will not deduct the tax.
It is the same process as any other lump distribution of an asset, like the sale of real estate—sell it from one and put it into another to equalize the cash flows.