Here is how to do this in TurboTax Online:
Using the Premium online edition, click on Federal on the left side of the online program screen
Click on Wages & Income
Scroll down to Retirement Plans and Social Security
On IRA, 401(k), Pension Plan Withdrawals (1099-R), click on Revisit
On the screen Review your 1099-R info, click on the button Add another 1099-R