Once you've filed, the IRS won't let you add anything to the original return. However, you can filed an amended return (1040X) and add the missing information. You'll need to wait until IRS processes the original return and issues any refund, though. Then you can prepare the amended return using TurboTax.
IRS only accepts amended returns on paper (no e-filing), so you'll print, sign, and mail it in.
Here's a link to the TurboTax support page for amending returns, which will get you started: https://turbotax.intuit.com/tax-tools/tax-tips/IRS-Tax-Return/Amending-Your-Income-Tax-Return/INF120...
