You cannot. You must file your 2017 W-2s on a separate return than your 2018 W-2s. You can file your 2018 tax return online, but you will need to purchase TurboTax 2017 to prepare last year's return. Please click the link below for information on purchasing this software.
https://turbotax.intuit.com/personal-taxes/past-years-products/
If you have already filed your 2017 tax return and need to add the W-2 form, you will need to file a 2017 amended tax return. Please view the link below for more information about filing an amended tax return.
https://turbotax.intuit.com/tax-tips/amend-return/how-to-correct-federal-tax-returns/L67t0F7il
[Edited 01/29/2018 I 2:10 PM]
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