To add a 2017 W-2 to the 2017 tax return that was already filed and accepted by the IRS, then you will have to amend the 2017 tax return.

See this TurboTax support FAQ for amending a 2017 tax return originally completed and filed using TurboTax - https://ttlc.intuit.com/questions/4252298-how-do-i-amend-a-2017-return-in-turbotax